Maybe try to explain his purpose in the band. If he doesn't like it tell him he should've picked a different instrument to play, and it's his fault his ego is hurting. Tell him your responsibility is to make the entire band sound good, not just him. You can give him a "bass only" mix for his own enjoyment if he wants. Tell him a good bass player knows how important he/she is, even though he/she may go unnoticed by the common listener. Make sure you have him sit in on a mixing session and mute and unmute the bass, so he can hear the entire ass drop out of the mix when he's not there. Say "Isn't it crazy how the guitars lose their heaviness and get all weak when you're not there? That's right, because you're the one making them sound heavy! You're the balls of the band! You may not get stroked as much, but when it comes down to it, there's no juice without you!"
Then bitch slap him.
