Basically, this dude kinda likes this other chick and you have a good reason to be questioning the situation.
Hands down, ask yourself seriously if this whole thing is worth it, and if you can trust him to begin with. Honestly, he sounds slightly skeezy if he's willing to just tell you straight up that some 17 year old (at the time) was the best sex he's had. Ultimately, this sounds like a one way road to relationshit and you need to get back on the highway of better situations.
Have a nice day, Bluename.