Fairly new step parent to autistic child who keeps damaging my house

Hi there, I have been with my partner for 2 years, he and his 10 (nearly 11) autistic son have lived with me in my home for almost a year and a half. His son and I get on very well in many ways, sharing a weird sense of humour and a love of going down strange conversational rabbit holes.

His son keep damaging things in my house - he smashed holes in a desk I let him use, so I now have no desk. He used a knife to shave pieces of wood off a chest of drawers in his room, and it is so destroyed that I can't even give it away - I tried to explain to him that some people can't even afford furniture and that it is not ok to destroy things like we are rich or like they are expendable. We don't have any spare money.

He gets large sticks and smashes other sticks in my garden but always does it among my plants, when there is plenty of room to do it elsewhere. He smashed a tile in on my bathroom wall, leading to a 10 day fix up job that involved many layers of filler, sanding, grout etc. He pulled the shower claw off the wall. He recently scratched marks into a freshly painted wall and claimed he could not remember doing it - his dad knows he did it, as do I. My sister bought me a Japanese maple for my 40th, he cut it with scissors and killed it. This is all done quietly when no one is looking.

We have been getting ever closer to moving in to a new house together, and I am going to rent my own house out, and so every bit of destruction makes this more difficult and expensive for me as I have to keep repairing things. His dad says he is doing it to get attention from him (his dad), not to upset me. I am very willing to accept that this is true, but it is difficult when the behaviour is all aimed at my house and my things. I guess part of the issue is that we live in my house and the furniture is mine etc so in a sense, most of what he has access to is mine. He however never damages his own things- there are no damaged gaming controller, consoles, or televisions because they are his and for his gaming, it seems many of you post about kids damaging things they use, but his own stuff is sacred to him, he only damages my stuff.

He ignores his dad when told not to destructive things and resumes them 30 seconds later. I don't know what I am posting on here for really, I guess I am wondering whether I just have to accept that I'll always be paying for damage he has caused, and some advice in learning to live with it and keep my temper, which I have entirely done until now, but it's getting difficult because I feel nervous in my own home and controlled by this.

We were all chilling out watching a programme he likes tonight and he swung a bottle opener (it opens like a butterfly knife which is why he wanted it, he is obsessed with blades and tools) around his fingers the entire time, ignoring his dad asking him not to because of the constant noise, and as he left the room, he dragged the sharp end of the bottle opener across my door.

His dad always taught him he does not HAVE to listen to someone just because they are an adult, and I actually agree with that, but his son interprets it as 'no one can tell me what to do unless they are a parent' - his son has said as much to me, that only parents can tell him anything (I actually think an 8 year-old can tell people what to do if what they are doing is stupid, so it's not about hierarchy for me). I'm the main breadwinner, I am doing up my own house through his destruction while we live in in, we are moving next week. I am also in the very early stages of a pregnancy that his son does not know about yet as we want to make sure everything is ok with the foetus before we disrupt his world with the news.

I want to have a good relationship with him, it was very good until a few months ago when enough damage had happened to my stuff only that I started to get upset and nervous in my home. I am struggling with the constant power struggle that he seems to be engaging in. He disrespects every adult, he refuses to share, he refuses to say please and thank you, he refuses to basic tasks himself and screams for his dad when he wants a drink that he could get for himself, he speaks to his dad like a piece of dirt, for example when his dad says anything to him he'll answer with "why would I care" or similar - that is the tip of the iceberg with how he speaks to his dad. The vast majority of things he says are negative or complaints, which is wearing on his father and on me. He smashes plants with sticks when we walk the dog, his dad tells him to stop but 30 seconds later he starts again, his dad tells him not to hit trees, he responds with its not a tree it's a bush, and he continues to lightly draw the stick across the trees and plants while looking sideways at his dad. He constantly points out different plants and asks if he can smash them. His dad let's him smash stinging nettles; I still hate the aggressive action and sound, it went on for 2.5 hours on the last walk I joined them on and it shot my nerves so badly I won't join them for a while as it is stressful and anxiety inducing. His dad told him this; the next walk involved constant requests to smash things. Obviously his dad takes the stick off him at times.

His dad is great parent who has his son with us 80% of the time because the mum is negligent - I have no issue with him being with us this much as I see it is best for him. He attends a mainstream school and is likely to be able to continue to do so, although with problems in one primary school who refused to accommodate him at all (they made him work alone in the corridor every day, it was disgusting) we jointly decided to take him out of school for a while for home schooling and it was the right decision. I would support that again if things went wrong again, but his new school are much more accommodating and allow him to wear hats and use fidget toys etc - they choose their battles well to accommodate Neurodiverse children from what I can see.

The more I type, the more I realise I have to learn to deal with this, I am only just starting to lose my temper with the situation (I do not shout at him, I ask him not to damage or use his tools on anything in my house) and I really want to find out how to deal with this better before our relationship is really damaged. I don't know what else to say - I would not be surprised if some elements of my post indicate my lack of understanding of autism and if any offence is caused. I apologise - it is not intended. I realise it may cause offence even if not intended, however, as you will all be parents of autistic children. I love and care for the lad, it is not nice to say this but the truth is that I can't have the same view of him that his dad has because I have not raised him his whole life, I've only known him two years, and one does not step straight into a parenting role - we have to get to know each other to develop whatever our relationship is, so it takes time.

I've never been involved in discipline as his dad wants his son and I to get on well, but I don't feel it's totally fair on me to not be able to speak up or take action, so I have started to do so very recently. I think if a tool is used for damage, it should be taken away, or if damage costs money, it should be paid for or contributed to by the aggressor even if they are a child - his father is more lenient and prefers to reason with him, but the behaviour resumes minutes or seconds later, it has no effect.

My partner is lovely and we only argue when my things are damaged again, and not every time at that. Is all of this typical for autistic children? What can I do to get better at this, apart from reach for the valium? Many thanks.