SUBREDDIT: r/relationships
TITLE: I [F21] am worried about my ex boyfriend [M21]. Acting out of character. What can I do?
POST: He and I were together for two years. I broke it off 6 weeks ago because although he's a great guy, I just don't see him as the guy I want to be with for the rest of my life. 

Since we broke up he's been partying a lot and sleeping with random girls - at least 2, possibly more. I don't really care, he can do what he wants, but I'm worried about him. When we were together he told me numerous times that he would only ever have sex with a girl that he had serious feelings for. We waited 3 months before having sex. He had only been with one other girl before me and that was his high school gf. In the time that we were together I think he went to one party and he never drank alcohol. Now he's doing all of those things.

He's acting very different and I'm worried about him. I know that we're not together and it's not my responsibility but I still care about him. What can I do? Should I talk to him?
TL;DR: 
I [F21] am worried about my ex boyfriend [M21]. Acting out of character. What can I do?