I feel for you and the poor little ones that are the brunt of his aggression. I have had my share of aggressive children over the years. The first thing that came to mind for me is that if the child is behind in his speach or other development and has a problem with his eyes and is smiling when he gets in trouble, he could be special needs and therefore would need to be treated or delt with differently. On the other hand, if he is in a house full of older children, he may get pushed around a lot at home and is taking out his frustration and lack of control out on the other little kids in your care.
I have a little girl that is always pushing the others around and grabbing etc. She is much better now but still has some off days. If she is having an especially difficult day then I make her stay near me so I can keep an eye on her or stop a behaviour before it happens. So when I am making lunch, she is not allowed to watch t.v. with the others she has to stay in the kitchen with me. She usually promises to keep her hands to herself and does.
You have to do what is right for you and the other kids in your daycare so if you think it`s a hopeless case or something you will not be able to handle then you need to move on. Hopefully the parents will realize there is an issue and deal with it.