It's better to be honest and upfront with him instead of stringing him along, which is probably very frustrating for the guy as he has no idea why he is being turned down for a solo. You would be doing him a favor by telling him why, so that he can possibly remedy the situation by taking voice lessons and other training that could help his singing. Obviously, he likes to sing and probably has no idea that he needs help in that department. Break it to him politely and gently, of course. If he reacts badly, then so be it, he has no right to do so. Sure it's a ministry activity, but everyone still wants it to be the best and singing has to be correct.