There are some kids whose physical threshholds are set differently from the norm. In other words, something that is just strong enough to register with them might be painful for another child -- so your son might not realize he's hurting another child. Is he under-sensitive in other ways as well? Like to temperature? Strong or spicy flavors?

Is it only physical roughness? Does he have a problem with empathy? Or is it emotional damage, like teasing and taunting also? Is he deliberately hurtful? Or does it appear more accidental?

Does he have trouble understanding verbal or non-verbal cues from the other kids that it's time to stop? Does he seem to understand other social cues? (boredom, discomfort, etc.)

Is it possible DS wants the other kids to go away and give him more space? Or is he sad when it happens? Is he surprised?

I guess what I'm trying to say is that you need to understand why he does it so you can find a way to change his behavior.

One tactic you might try is called Applied Behavior Analysis. (It's used a lot in families with autistic children, whose behavior is less predictable using conventional rules. You have to try to understand the world from their viewpoints, because it isn't like ours...) Try to be very aware when he's around other children in situations where this might happen. You want to be able to write down what was happening before he acted out (very specifically), exactly what he did, then what happened afterward (who did what) and how he responded next.

After recording a few of these incidents, try to see what the common threads are.