It sounds like he probably just needs relationship advice. What better person to go to to vent about exes and get advice is a person in a successful, long-term, happy relationship?

People generally downplay their exes when they're interested in romance with a new person-- they don't go on hourly rants and dissect their life. It sounds like he is just sad. I went through a breakup recently and having friends to chat it out with (some who I hadn't spoken to in a long while) really does help. Especially friends who are in their own happy relationships rather than single ones, the advice my single friends have given has always been drastically different from the ones in relationships.

I wouldn't overthink it, but definitely talk to your girlfriend that you are feeling a bit insecure over their amount of contact and say you'd like some reassurance. It's normal to feel insecure sometimes but what I've learned the hard way is that a healthy, loving partner will do their best to reassure you in those times rather than to be upset or suggest you do not trust her or other things that we see often. So far it seems she's been open and honest about the communication, give her the opportunity to reassure you! :)