Ok, tonight he asked, and would not let it go. I went with: I really like the music, I find your guitar solos give a lovely atmosphere, I really enjoy your instrumentals.... And he just kept asking.
At this point he knew something was up. He just looked at me with one raised eyebrow and told me to cut to it and tell him what I thought of his singing.
Then something new happened, he said he knew he wasn't any good at it, but did not know how else to get stuff off his chest! I told him I think he's particularly soulful on a bass guitar, as in really really good. He can tell the story he wants to tell with that instrument. He's also good at mixing audio, so a combination of his strong points has real potential. He learns fast, so it would be a really rewarding learning curve for him, and I'd be proud to hear him play that in front of others. He tried out some things I suggested, very much the dumb leading the deaf, but it worked! He jammed his behind off! It was great fun, and he noticed a drastic difference in quality himself!
I'm so glad I asked all of you!!! It was so much easier to have all the kind, but truthful suggestions at hand when he looked at me with his big blue eyes and told me to tell him what I really thought.
He said he was sick of people just telling him everything was "good" (his parents, and some of his friends), and he respected my opinion because he felt I gave him my objective view of his work. Thank you ehellers! Thank you, thank you thank you!!!!