I'm sorry you're going through this. I have gone through something similar with a Pisces. I agree with what other people have said, maybe it was too soon after his ex that he met you- it's possible that you could've been a rebound or that he still may be wounded from the break-up. Who knows, maybe he saw something in you that he didn't like or that scared him.
What I learned from my situation is that sometimes there is no explanation. I've woken up some days and have (out of the blue) felt differently about someone or something. Furthermore, I've had experiences where I've known that someone was perfect for me, but I just didn't FEEL anything.
To answer your question though, yes, they do come back. I dated one 6 mo. ago and I ended things because he wasn't ready for a relationship. A couple of months later he started pursuing me again, taking me out on dates and texting me non-stop, so we decided to try again. I notice they are extremely aggressive in the beginning, but then they back away for whatever reason. The second time around he still didn't want a relationship, but we've remained friends (occasionally with benefits lol).
I don't know if this is a Pisces trait, but he told me once after we were through that he doesn't take rejection well and that he "wants what he can't have." In retrospect, I guess I made it too easy for him- I tend to be extremely straight forward when it comes to relationships. I don't like leaving anyone in the dark and I tend to be very honest. This probably scared him away or took "the chase" away. He said one big reason why he didn't want to pursue anything with me was he didn't think enough time has passed since my last relationship.
My guy friends tell me that when a guy really likes/loves you, they're going to do whatever they can to be with you if they see that you're worth it. Would you ever ask him what went wrong? You owe it to yourself to know, just for closure. Get him drunk! That's when all the truth comes spilling out of them, trust me lol.
Be well and best of luck.