I am not going to say "once a cheater, always a cheater", because I know that that isn't always the case. The way of confronting him is really simple. It has to be done with tact, and it can be.
I'd suggest going for coffee or something, in a real quiet yet public spot where you can talk. If you trust that he will not become violent, it can be done in a more private setting.... but let him know ahead of time that you have something very important that you want to talk about.
BE right up front. Let him know your concerns about this behavior, pulling no punches. Tell him that you are scared or uneasy with his fixation with younger females, and you feel as if it is going to be a problem with your relationship. (I'd suggest that he may say one of 2 things.... either it is just a problem for you, or that it is really nothing) The problem with that is, it "is a problem" and if it "is a problem just with you" it is certainly worth addressing! You need to know and mention that you are involved here and this makes you feel real uncomfortable.... not insecure, uncomfortable.
Obviously this mans character is in question. I don't know if he can say anything to make you change your mind.... in fact, I doubt it. But by addressing this, you can find out whether he is indeed addicted to this behavior..... He also has proven to be a cheater, so you do have to keep that in mind.
The decision is yours, but addressing this is simple. They say breaking up is hard to do, and I agree. What's harder is keeping yourself in a situation that you don't like and trying to pretend that nothing is wrong, and not saying anything about it. You owe yourself at least that.
I am going thru the same exact thing with my boyfriend only differences is he didnt cheat on his wife she cheated on him and left him for someone else but my boyfriend doesnt give me enough sex but i thinks its because he masterbates to porno or pictures of young and i mean young girls. i found his camera and it had 500 pictures of young girls that walk by his house on there way home from school and pictures of his neighbors butts while they are walking into there homes.. this happened 2 months ago i broke up with him and he begged me to take him back and stupid me did, i told him that things would never be the same and they havent been infact i used to trust him with everything now i dont believe a thing he says it really ***** because he treats me like a princess and is always worried about me and caters to me if i am sick he will be right by myside when i was in the hospital and someone had to care for me he insisted i stay at his house but his only problems is young barely legal girls and the icer is i have a 14 year old daughter who i refuse to let around him since i found that camera
Dump him. And have sex with a younger man yourself, then show him pics/video of the event.
Be sure to mention every way in which the other man was better than him. In other words, disrespect and humiliate him. After all, that's basically what he's doing to you. Never buy the male line about how they are controlled by Mr. Happy. Tell him you are controlled by your heart, which says he is a pig! Oh...and THEN, dump him!
I'm a big believer that dating is a time period in which we gather facts to see if this is a person we want to be with longterm. You mention a few things that make me nervous about your boyfriend. He's a cheater, for one. Whether it was young women or not, he's a bonafide cheater. That would be a problem for me, right there. And you mention multiple affairs and an affair with a trusted person close to the family (babysitter). That is not a man of character in any way. You will age as will he. You don't care . . . but he may. I wouldn't risk it with this guy. Find someone that doesn't have a history of lying and cheating and being consumed with youth. Find someone that you can really grow old with and you know will love you no matter what. That is my honest opinion. goodluck