Your son can apply for the Discover it® for Students credit card. There is no annual fee. Every purchase on the card will get a 1% cash back reward. Also, there are rotating categories which earn 5% cash back. Credit activity will be reported to the major credit bureaus, so your son will be able to build his credit history.
Another option for students without a credit history is to apply for a credit card with a co-signer. Discover and Chase offer credit cards that require co-signers.