Hi OP - you've had a mixed bag of responses here from agreeing with you to hailing you as judgmental .. as a parent of w boy like the one 'striking' yours may I offer my input?
My middle son has ADHD, learning disabilities, traits of autism (no formal diagnosis as yet) a developmental delay of about 3/4 years (he's 8 but socially and emotionally about 4/5), he has sensory processing disorder and a language disorder.
He is very socially awkward - has literally no idea how to manage friendships or engage with peers appropriately - admittedly he's improved enormously this academic year and we have seen a down turn in undesired behaviour.
I think a break down might help ..
ADHD - he's very distracted and unfocused, sitting still is hard going and more so is impulse control.. if he is being pushed by an adult to focus on a task and he's reaching his capacity to cope and another child is doing something like 'tapping a pencil' or their foot or something absent minded, my son would find that beyond impossible to ignore .. his impulse control issues would likely lead to him just doing the first thing he thinks of to eliminate that distraction. Often that would be to strike or shout at that child. He immediately knows he's done wrong and becomes very emotional, again unable to control any impulses that comes with that huge emotion.
We have worked very hard with this now and he is getting better at recognising when he might act inappropriately and he simply gets up and walks away.. maybe 50% of the time.
His autistic traits - he hand flaps, a lot .. it's a 'stim' or 'self stimulation movement'. It can be quite extreme and if you're near by you'll
Likely get a whack .. he isn't aiming to hit you but the need to flap is so great he cannot control it.
Sensory processing disorder - so he's super sensory seeking .. orally especially and 'biting' or mouthing was an issue when he was 3/4/5 and it was never with any ill intent but always because as a small baby does, he wanted to explore everything with his mouth. Now he's older he is very fixated with drumming and tapping on things .. everything.. to establish the different sounds and feelings items feed back .. including people .. again he isn't 'hitting' anyone for the sake of violence but because he's distracted and has limited impulse control and the urge To know what percussion he can get out of you wins out a lot of the time! He also get a big sensory feed back from running hard and fast into solid objects .. sometimes including people .. he can't explain why sadly.
Language disorder - he's ability to verbally communicate is quite delayed also and resulting in frustration and misunderstandings which again down to impulse control.. does some times lead to lashing out. Also he seems very abrupt or unkind with his words but he's struggling to communicate and is extremely literal .. black and white to his core.
It would be easy as a parent of a child in his class to perceive his behaviour as bullying but the truth is he hasn't the capacity to bully anyone or the awareness of others for the notion to even occur to him. I suspect the same is for this child your son is being incredibly patient with .. your son is a true credit to you and a pure soul.. my only wish is that every SEN child goes through school with a class mate like your son.. please keep encouraging his mind set and should he ever wind up actually seriously hurt please don't shut down the friendship in knee jerk.
I will never be able To explain to you what that friendship will mean to that SN child and his parents but please know they would do anything to work with you in maintaining a friendship with what clearly seems to be a very compassionate and sweet child such as yours ❤️