I have 2 five yr old boys, one my son, and although it's not that bad, they do act up when they are together. I simply seperate them. If DCB is not going to behave the way we behave in my home then he will sit in the chair until mom comes. If he continues, the chair get's turned towards the wall, if he continues mom is called or emailed with the concerns.
If my son is the problem, he simply spends the remainder of daycare in his room. Children are not allowed to behave like that in my daycare and if my son wants to be a part of it, he better behave the way I expect.

I find the key to success is to not look like it effects you. Sounds like he knows how to push your buttons.