He probably doesn’t even realize how much of a nuisance he is being, thinks he is actually great. Depending on your personality, I’d either recommend calling him on his BS relentlessly, or ignoring him completely, if you do not handle stressfull situations well. Me, I’d choose option one, there is a risk though that you might become as cynical and pain-in-the-ass-ey as your flatmate, so be wary of the abyss looking into you ;-) Good luck!