I agree with Dan W. If this guy is a well known stroker then tell the company. They won't appreciate sitting next to them for nine hours anymore than you will.
I was in the bar with one management pilot who was complaining about the behaviour and attitude of a new bod. When i mentioned that he was an arse at our previous company and was suprised he made it in he rounded on me and said 'well why the hell didn't anyone tell us?' People can act very differently at an interview and it normally all comes out on ground school. By then it's too difficult to get rid of them unless they're incompetent.
If it's a personal issue you have with him then it's a little different. However, if it's going to make it difficult for you to fly together (ie 3/4 crew sectors) then i'd do something about it. You joined the company first, it's just tough on him. That's why aviation is known as such a small industry and should try not to upset people!