At face value, I would be leery of letting a teen play in a game with me; but is it really any worse than having them play a video game? If they won't, at least, let your son try his hand at RP, then they are prejudiced wrongly.
If, on the other hand, the players feel you might be playing favorites with your son, then they are prejudiced correctly.
I would recommend one of the players in your game where your son plays has a chat with The "problem" group about what your son brings to the table. A neutral opinion may hold more sway than you saying "My son is totally a great role player. Pinky swear."
Tabletop RPGs are cooperative games, and as such, without the cooperation of our pcs, there is no game.
If you can't find out why they have reservations, and if they flat out refuse to give your son (who wants to RP! Let him!) a chance, I would remind them that blood is thicker than water.