I agree, just don't leave them alone.
Also, since he's nonverbal (or at least poorly verbal) I would do show-and-tell. For hitting, if he wants to play or touch someone, gently take his hand and pat or stroke the person or dog. After you guide his hand, have him do it himself with your hand hovering about 1/2" over his, in case he gets rough or inappropriate. A professional therapist will be able to give you more tips.
Also, allow the other kids to get a little rougher than normal in response, to show their displeasure. My easy child was born a peaceful soul, and I had to teach her to be assertive and sometimes, rough. (She ended up pulling a few stunts of her own, but that's another story!)
I don't know if your difficult child is too developmentally delayed for this, but I met a toddler once, in easy child's daycare, who hit all the other babies and toddlers. The daycare provider spent most of her day keeping him away from them and yelling "No!" This was long before difficult child came along, but I instinctively wanted him to hear "yes" and know what to do, in addition to what not to do. So I sat down with him and asked him, "Do you want to play with the baby?"
He nodded yes, enthusiastically. I held his hand and in a soothing voice said, "Nice baby, nice touch."
The baby was fine. A tough little cookie, whether being hit or stroked or ignored. Probably a golden retriever in another life, lol!
Anyway, it worked, for the few minutes I was there.
I could see that a lot of work needed to be done.
I always wondered what happened to that toddler.
I feel for you!