you need to deal with a biter by shadowing which means the child needs to stay beside you so that you are right there when he treatens to do it again. That means if you go to the bathroom he sits outside the door, if you are making lunch he reads a book sitting beside the fridge. He loses the privilege totally of playing alone in the playroom because he "hurts his friends". He will understand the word hurt. I don't usually bother with the apology part since it becomes just a routine and in a child's mind becomes a if I say sorry it doesn't matter. One of the biggest reasons for biting is lack of language. Remind him he needs to ask you for help if his friend doesn't understand him or has something he wants. But it is a phase that will pass so your job is to stay between him and the other kids which means prevention rather than only reacting after the fact. It does tend to be a short lived phase. Make sure the child is getting enough sleep and drinks. A thirsty child is a little menace,lol moreso than hunger. Since you sense it is from frustration work on vocabulary naming things in the room with the child.