I get a similar thing from my boyfriend.
He sometimes thinks that while I'm at uni, I'm going to find someone better than him, and cheat on him, or split up with him.
I'm fairly sure we talked(well texted) this through before Christmas, and everythings fine now.
I hope he realises now, that he is perfect for me and I don't want anyone else!
So talking it through helped us, but you said you've already done that. Maybe talk again, reassure him. I've had to tell mine loads of times, over the course of our relationship, that I don't want anyone else. Just talk it through with him, whenever it gets brought up, I guess is what I'm trying to say.
Hope everything works out for you!