I think it's a neat idea (depending on the theme of his room), but you may be right. His room may be not dark enough for it to really show. Does he have darkening blinds?
Some reason I can't get into the link? But when I was in jr high I put up stars & moon in my room & I loved it. It had a nice glow when it was dark but you think about it most of the time when you are in your bedroom in the dark your eyes are going to be closed. So I don't know how extensive the one you want to do is but that might be a consideration. If it is just peel & stick where you want it was fun.
Re: Anyone do a glow-in-the-dark ceiling mural?