He got misled by a priest. It will probably take another priest to help him on that point. Honestly, it is somewhat fishy. There must be another story behind it. My recommendation is not to judge him on it, but see if he is open to talk to another priest to find out about the truth. If he is right, he got nothing to lose.
Meanwhile, love him the best you can. Encourage him to love God and serve others more. Only love can draw someone deeper in Christ, and only the light of Christ can entice him to want to know the truth.