I find it hard to believe that this behaviour would ONLY be seen while under your care. Either mom and dad choose not to see it (happens aaalllllll the time!!!) or, as playfelt said; daddy or someone is teaching rough and tough play methods.
What type of situations is this occurring in? I have a little boy that will hit for no reason (or push, etc) when he is tired or mad. He tried to bite me, for crikey sake, because he didn't want to hold my hand in a parking lot. No matter what, my consequences are the same. But I do keep extra watch on him if I think he might be in the "danger zone", so to speak. Then I keep him close to me, so that there is less likelihood of an incident.
It's not out of the ordinary for his age group, but it absolutely needs to be handled.