Any advice on a very aggressive 10 year old?

Hi, my son has been diagnosed with ASD since the age of 2. We have overcome many challenges over the years, however, the biggest challenge that we are still struggling with is his behaviour.

He is constantly hitting, slapping, hurting himself (banging his head) kicking the walls, slamming the door and it is getting much more difficult to manage as he gets older. The bigger he gets, the more painful it is to deal with his behaviour.

We have been to many workshops and sought advice from teachers etc regarding his behaviour. And some strategies has been used (such as the ABC method). But, what we're struggling with mainly is what to do when he is hitting and punching. 

This is what we are currently doing: when he has his tantrums/ outbursts, we try and figure out what the problem is and then we try and calm him down as best as possible (e.g giving sensory toys, blowing bubbles). But during his outbursts, he has been hitting others and screaming- so once he has calmed down, we set a timer as a way of discipline and to make sure he understands that he cannot be hitting to get what he wants. 

This does work, and some days are easier than others but the main issue I have is that he will still be violent towards others. I am not sure how I can deal with this. I make sure nobody is around him when he is hitting, but then he resorts to kicking the wall- there is no way in removing the walls away from him. And he also slams the door so hard that he has already broken 2 doors. He will bang his head and when I try to move him away from anything dangerous, he attacks me. The more older he is getting, the more difficult it is getting to manage his behaviour. My deepest concern was when he came back from school today and I had been told he had attacked one of the staff and made his nose bleed. I was very disappointed and ashamed when I had heard this and I am fearing that it's only going to get worse.

I am trying my best but I feel like nothing is working. He will keep being violent, and I feel like the consequences; i.e placing him on a timer or not allowing him to go on his tablet, is not working effectively and I am struggling to find a solution. It sometimes gets too much for me, especially when you've already had a stressful day. I would really appreciate any help or advice to help me deal with my child better. I just want all of the violence to stop, or at least be handled better.