In the event he acquired his U.S. Passport by fraudulent misrepresentations, it can be revoked and his U.S. citizenship would become invalidated, regardless of claims about naturalized versus natural born citizenship.
Actually, in that case while he may have his passport revoked, more would be required to for citizenship to be forfeited. There are many who are citizens who don't hold valid passports.
Though the conditional nature of your statement begs the question: to obtain a passport he would have needed to submit documentation attesting he was a citizen. Obviously, the passport agency found the documentation submitted to be valid.