Hitting toddler

Hi I'm new here.. my little boy well I say lttle but he's a big boy now he's three,

We are currently waiting for a asd assesment. 

Been finding it hard lately with his ongoing aggression towards his older sister, me and other children. He has very limited speech but is coming on leaps and bounds since starting nursery. Finding it hard to socialise with other people due to his hitting. 

When he hits from my point of view it looks like he does it for personal satisfaction if that makes sense. I'm losing my mind trying to understand why he is doing this and how to approach this. I understand that lack of communication is normally the reason.

When he hits he hits on the back with a strait hand very hard and seems to tense up when he is doing it. This is also leading to him being hurt back by other children and I can't help but feel sad for him as I don't see it as if he can help it if that makes sense.

Anyone have any advice or experience in this sort of behaviour ?

Thanks