I think it's normal but I think it could be a problem too. If I were you I'd talk to him about the danger of objectifying women. Tell him that it is normal for him to be attracted to the opposite sex but that it's not normal behavior for these women who are in porn and for the men that look at it. Tell him it's lust and fantasy, not the real deal. Some day he's going to meet a woman and fall in love and she will respect him more if he respects her enough to avoid this kind of behavior.