I'm with Dong on this one, you're an adult, you can tell him politely that he's getting on your nerves. If you let it go on and on what is likely to happen is that at some point you're going to lose your temper, call him a huge fucking knob and then you'll end up falling out and still having to work with him. Much better while you're still relatively calm about it to take him to one side and say 'look I know you're having problems with the boss and I don't mind you venting once in a while but you're doing it constantly and this is a problem for me'.