Your boss probably went to you because he considers you to the "bigger" person, more likely to behave like an adult when dealing with difficult situations. He must know that it's putting a strain on the whole team, and only asked you because he knows Peter is a tw4t!
I know it must be difficult to swallow your resentment again, but it can't go on forever and you have to try again, as hard as it is! Go for it, sit him down, and explain the situation, and say that whatever your feelings towards each other are, it is a work situation, and you have to both behave like adults, and put your differences aside for the good of all.
If that fails, put laxatives in his tea.