There's a difference between being a musician and a singer - If someone cannot inherently sing, then they cannot sing but you can pretty much teach anyone to play an instrument, no matter how little knowledge of the instrument they have.
Teaching someone who simply can't sing, well, it'd be like trying to teach a cock to lay eggs! Pointless and painful lol.
I'd be honest with him and explain that whilst you find his passion for singing sexy as hell, he should maybe focus on a particular instrument other than his voice as you think he'd suit it better or something of that 'ilk' lol. Perhaps say you find guitar players sexy or something - appeal to his inner sexiness and try and persuade him that he'd make a better bass player, or drummer or something lol.