If he can really sing, but can't play good guitar.explainto him that he can really sing but can't play guitar. So just sing and play tamborine or something. If he wants to be in the band he'll understand. Remind him of Mick, Morrison etc. If he cops an attitude, put an ad in the paper for another singer. Maybe the next one will play rhythm.
We had to tell our singer not to play guitar; now he sings much better because he dosen't have to concentrate on playing guitar.

Guitars save lives!!!

PS mAYBE YOU SHOULD SING!!