Be the first to raise it at the next rehearsal,but do it before you start playing. Don't shout or get drawn into any childish regiment though, this will only reflect poorly on you. Work out in advance what you want to say , and how you are going to word it.
Remember this is your band and you need to cherish it, bands are about more than strapping a few chords together.
Let him be the first to shout / swear / throw stuff / poo in a corner etc... it will mean that he's already lost, and the rest is tying up loose ends. Chances are the others everyone is harbouring doubts about this interloper, but us blokes will generally do anything for a quiet life, and it sounds like the band are just hoping it will sorry itself out.
It won't.
If he does insist on playing with you then be the bigger person; be as nice as you can to him, starting with lending him your lead, I recommend one of these:
[attachment=114610:484106_10150926981843097_69291677_n.jpg]