Even though you claim him as a dependent, he must still file his own tax return if he had earned (W-2) income over $6,300; or self-employed (1099-MISC) net income over $400; or unearned income (interest, dividends, capital gains) over $1,050.
Even if his income is below these thresholds, he'd still have to file a return to get a refund of any withheld taxes.
IMPORTANT: If he does file a tax return, be sure he checks the box on it that someone else can claim him as a dependent.