I'm sure he cares a great deal about you, or else he wouldn't have been up front and honest about the whole situation. I also don't think just because people date for a few months it means that if their ex pops back into their life, they hop over back. Depends on the person, situation, and most importantly how the planets are aligned /nods.
I guess its always best to put yourself in their shoes and look through their eyes and keep an open mind.