He still might regret it. Don't really see any reason that would be good for your future. Even if we are ignoring the huge red flag on him going after people so young. Usually control of some sort is involved. Young women tend to be much more susceptible to manipulation then the people his age. Financial aid provides control. I have no explanation other than that which makes any sense. Or his still desperately in love with her.

And yeah hiding this from you for so long means he has the mindset to hide serious stuff from you. And texts etc can be deleted and edited to fit a story, not saying that is happening but everything is kinda suspect after something this big

His paying for her attention. And probably gets some control or something in return. Or he still loves her. No good options for you really. And definitely not since he hid it. And how sure are you that he left her and not the other way around? That would make more sense.