There are some people who are listeners. They're the folks you have to cajole, encourage and threaten to get them to either play at all, or play loud enough to be heard. When they do play, they'll find ways to fit into the music. They'll listen to what others are playing and what they're playing and adapt. There are some who won't listen, and I don't quite understand them. They don't care if they fit in, they only care if they're heard. I don't know, maybe they think everyone else will adapt to them. Maybe they don't know that music is about communication, not exhibition. It doesn't matter what instrument someone plays, or if they sing and don't play - this type of individual can be a real pain in the ass.

I don't know if they have the capability of learning - I've never seen it happen myself. I'd always vote for blunt but kind honesty first. People often wait too long to say something and get mad and blurt something out. What about "Are you aware you're playing faster than anyone else? Would you please slow down a bit?" If that doesn't work, simply "slow down" during the tune. If that still doesn't work, take the fellow aside and let him know that he can choose to play with everyone else, or not play. And if it still doesn't work, and you have the backing of other session musicians, you may need to simply tell him to leave.