Guys, it's not about "letting him try", he's happy with his (very complex) role in the band.
Some of you are making it sound like he's being held back... he isn't. It's nothing like JP not wanting another guitarist and I'd bet that if JR suddenly said "hey, I'd like to do some background vocals" they'd be all over it.
Would *I* like him doing backups? Yes, I would, cuz even soft/quiet vocals helps them sound fuller, even if they're not the "best" backup vocals.