How did you get your hands on a demo? Did you do this in a sneaky, dishonest manner, or did he offer it to you? did he ask for your opinion? Are you a music critic?
I am not sure what to tell you to do here. I think this could be very damaging to your relationship. Is your relationship strong enough to withstand you telling him that you think he cannot do well that which he wants most to do?
Anyone else have definite opinions on this one?
chickita