I told him multiple times that if he was going to try and do his own thing, he won't be participating with the group, and the group is the entire focus of the game.

I suppose I could have made it more explicit that he could join the group or he could leave the game.

I should add that that was many games ago, and he has since begun participating, although he often tries to go his own way and threatens to leave the group constantly, but so far he hasn't actually tried leaving the group unless it was agreed upon for strategy reasons. (they split up inside a crypt in the most horror movie fashion possible)