His mother claims him as a dependent.
He files his own. He will more than likely get back any income tax he paid in, unless he made more than about $3000.
Make sure he checks the box on his return that indicates that someone else is claiming him as a dependent.
This is VERY important!
There may be extra calculations since he is being claimed by his mother, so he may want to have it done by a professional. If not, make sure he reads all the instructions that pertain to him.
Snipes