I think my 20 month old is autistic and I’m finding it hard to find support and guidance. Really overwhelmed right now.

First to address a few things! Just buy one halogen lamp for his room and in the evening, just use this. LEDs are damaging and can feel oppressive to those with light sensitivity as the band of light is unnatural. The best analogy I can give you would be like filtering a song. Take out all the bass and make the high end piercing. The expression of sonic torture is: It makes my ears bleed. This is how difficult it is for the eyes to deal with light which isn't naturally glowing off a heat source (such as the sun, fire or the filament of an incandescent or halogen bulb) Have a look at LightAware.org

Second, I want to stress he IS communicating. Language isn't only verbal. We say music 'spoke' to us or a painting or a ballet. Humans haven't always used verbal words, but they've still been perfectly capable of designing boats, hunting, crafting tools, clothing, building and collective togetherness. The Autistic that senses an internal difficulty accessing words (most of us do when stressed), might actually be incredibly skilled at other things which most aren't - map making or maths, music or engineering. Words are only one tool of many. Feel free to expand your vocabulary. Children thrive when we as parents are dependable, open, ask how we can help or present ideas for help, when we take responsibly for our faults and always seek their trust (a thing easily broken and not easily mended). 

He might be helping you clap because there is something enjoyable about the sound. Or there is something to you allowing his request and this is the way he's learned you'll respond with openness. 

My grandmother had instructed me to never say no when my son was born. She said it was unnecessary. Just redirect. Just be there. I'd offer my son different things and it would give him a little sense of being in control to release what I didn't want him to have and take something offered. 

Now, as for the seizure and the screaming through the night - it could be important to have him tested for epilepsy, as we're prone to it. And then I might try changing a few things. First is his diet. Most of us have gut issues. And even when things feel a little more normal, I still have to mind I can never have high-fibre veg (no brassicas like spinach and broccoli), absolutely no beans/pulses/legumes, which includes the peanut and no grains except white rice and GF oats. I'm completely Gluten free but also working with a low FODMAP diet .

Also, I might avoid digital media as much as possible as screens and flashing elements can create headaches. Possibly make sure his surroundings aren't unnatural fibres (polyesters are made from petroleum, so it's essentially a form of plastic) such as 100% cotton and wool. Tree fibres are only good for summer as our biology responds to them and cools itself down. Wool and cotton are biology friendly. I'd also make sure there's no chemically scented candles, plug-ins, cleaning agents like fabric softeners, detergents and such. These can interact with the gut and cause severe head aches as well as stomach cramps. 

Toe-walking is, to be fair, incredibly enjoyable. Female Autists get away with it more than males as no one thinks twice about heels. Make sure he gets to run and exercise enough. Running as sport and a proper stretch daily can help relieve the need to do this.

That 2 year wait may prove to inadvertently be more helpful tbh, after all he's not yet 2 yrs old and developmental milestones often aren't a clockwork affair. If we handed out autism diagnosis to anyone who was even remotely delayed before preschool then the autistic population would be nearly 8/10 My kid showed no signs of being neurodivergent until reception year at school.

NAS85272 said:

Ds never has communicated with us but has always been a relatively happy little soul, laughing and smiling but now has what I’m guessing are autistic meltdowns? He will scream so loud and almost angrily for hours through the night and there’s nothing that seems to soothe him, hugging /food drink/ tv/games , even ipad just thrown they are so scary, he will also bang his head throwing himself around, pull my hair, grab my face and bite me. They are so loud the neighbors came round.

That sounds like a regular tantrum to me, seen it in loads of 1-4 olds and it's the most standard thing in the world because all kids that age lack adequate communication skills to express themselves rationally.

NAS85272 said:

I’m so sad for him and worried for his future , I feel like I’m not doing anything right but can’t find support to tell me how to help him, my health visitor just told me I must advocate for my son and let him lead me.

Don't feel sad, he doesn't feel sad for himself and he won't all the time you don't compare him to his peers as though he is lacking. He's just himself at this age, not in competition with any other kids. At this stage you are doing great just to get enough sleep and keep him clean, fed, healthy, and happy. It's tough raising any kid under 2 yrs old so give yourself some credit. You need not worry so much until he is much older unless his progress halts entirely. Otherwise progress even slow is still progress and little victories are still victories. 

By all means set it in motion so that he can be seen by someone at the end of 2 years now, but until it's his turn on the waiting list you may not know the full extent of what is occurring yet anyway. In which case overly worrying isn't going to actually solve anything.

Keep calm and keep going mamma you're doing well.