He used to be in a talking stage with a 14-year-old and let her hit on him when he was 18

Hi everyone, pardon my English since it is not my first language!

I've been talking to this guy online for almost a year now and we're in a stage where we simply are a couple unofficially until he got here, which he planned to by the end of this year. Three months ago, we talked about one of his close friends who he claimed to meet her online 4 years ago when he was 18 and she was 14. He revealed that at the time they met she was going through a lot due to her abusive boyfriend that he refers as a "groomer," and as a friend he decided to console her as best as he can. After that, the girl told him that she liked him but do not want to break up with her bf and be with him. He told me he did not have any feelings for her until she confessed that, so he chose to "be there for her" for about half of a year later. During these months, she broke up with her "groomer" bf and moved on with someone else, and once again told him that she liked him again, then starting to hit on him. Yet she still wanted to keep him around and dating somebody else, and at this point he decided to stop liking her. 

Those are what he told me, and I don't know if it is entirely true as he claimed it is. I was truly concerned about his story and asked if he knew what he was doing with a minor when he was no longer one. But for some reasons I was blinded with his words and the claims that he is no longer in touch with her and has refused to meet her in person (which she asked for last December, she was 18 by then) and somehow forgot about this completely. I only got reminded about it this morning when I watched a video of Jidion catching a pedo. I asked him again and he immediately panicked, then started apologising profusely for not drawing a boundary with that girl when she was still a minor. What concerned me even more now is him saying that

"I'm genuinely grateful that i did not damage her"

"I still feel uncomfortable talking about this but I will because I feel really apologetic" 

"I know I shouldnt blame anyone but my friends who knew about it did not point it out for me or pull me away from it"

"I was 18 and fresh out of high school, i was naive and stupid"

I know I should not continue with him, but part of me feels distraught thinking of leaving him since he always tells me how lonely and sad he is, that he has almost nobody in his life except for me and a few of his friends, and that his parents were sick and was not there to help me emotionally. 

What should I say and do to him now, I dont want to hurt him but the thoughts of staying and doing something sensually with him disgust me. I'm also worried that if I escalate things he will expose the explicit I sent him, although my face is not included in all of them. TIA!

TL;DR

I've been talking to a guy online for almost a year and planned to date officially when he visits. He told me that 4 years ago when he was 18, he was in talking stage with a minor twice in six months and allowing her to hit on him, though they did not meet in real life. I don't know how to cut off with him without making him mad and decide to expose my explicit photos.