Is it a musical theater production? If so, I'd advise that he focus on his character and why the character is singing this particular song, and how it fits into the story--who is he singing to, and why?
If it's a solo in a choir concert, I'd say to focus on how much he enjoys the song itself, and how he'll feel about sharing the song with his friends and family.
Either way, I would definitely advise that he NOT think about things going wrong. That's a great way to ensure that something goes wrong. Instead, focus on how much he enjoys the song, and remember that he's performing because it's FUN.
None of this, of course, is a substitute for practicing, practicing, practicing until he knows the song so well he can perform it in his sleep. Sing it in the shower, sing it in the car, sing it whenever you get a chance. If any of his brothers and sisters complain, tell them to cut him some slack. And be prepared to put up with it yourself. :)