Lots of good advice above, but the one I'd stress is tell him to put down the guitar and practice singing, listening just as intently to the sound he makes as he would to the sound of the guitar strings. Recording and playing back might help. If he feels out of his depth,he could go right back to basics and try something really simple like Go Tell Aunt Rhody, and move on to Happy Birthday.
And I do think a few lessons with the right sort of teacher could help.

Marje