Does he have any credit cards in his own name yet? Has he been an authorized user on any of your credit cards for any length of time? What is his FICO score? Does he have a job -- even part-time?

The best way to qualify for the cards with the most lucrative sign-up bonuses is to have a good credit history, and decent income. To help build his credit history, being an authorized user on someone else's credit card will help. In addition, department-store credit cards might be relatively easy to get, as well as certain "student" credit cards. Does your son's college have tie-ins with any credit-card issuers looking to sign-up students?