Your son can apply for the Discover it® Student Cash Back 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 (up to the quarterly maximum). You have to register each quarter to get this bonus. Credit activity will be reported to the major credit bureaus, so your son will be able to build his credit history.
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All rates and fees, and other terms and conditions of the products mentioned in this article/post are actual as of the last update date but are subject to change. See the current products' Terms & Conditions on the issuing banks' websites.