Everything written above is true in the fact that:
1.it is wrong
2.there is a history of this behavior in his family and his associations to sex are slightly screwed up
3.IT NEEDS TO BE HANDLED IN A delicate and quite matter.
The reason I say delicate is you both seem young and still maturing mentally as far as sex and self actualization is concerned. You also have a relationship that you seem to be invested in emotionally and mentally, given you having problems with your "find". So to save him from being put on a predators list I would talk to him about seeking treatment for this, so long as it does not hurt (legally label him a predator) him in the future.
you have to realize that even though this is sick and wrong its the human mind. We all think things that we some times feel ashamed of and this is just part of the human condition. The difference between being and thinking is accepting the idea as OK. In essence, you are not your thoughts, just your accepted ideas.
For him, his thoughts have manifested into visuals. The snakes head must be cut off here to keep it form turning into ACTIONS. This does not mean RASH action on your part should be taken. you should really establish a line of communication and questioning to determine the severity of this situation. Maybe they are just really young looking girls. regardless, this is a problem and evidence of it not being one needs to be found.
Don't listen to these people who are saying "leave him". If you are as invested as you sound into this relationship it is an obligation to help him.
best of luck.