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Check his drivers license or birth certificate. If you know a good friend of his that he possibly grew up with you could ask them if they know his true age or ask a family member who better would know his age than family. His driver's license would be your best bet because he has to have his true age on it.
Usually to find out someone's age online you have to pay money for it and also if he is truly underage it may be hard to find out what his true age is.