What should I talk to him about to keep a good convo going?

Talk about the stuff you like to do or whatever comes to mind. Ask him what he's done since you spoke last, and tell him about what you've done. If you have some funny stories, then be sure to tell those. You can pretty much talk about whatever. If you're both interested in eachother, it doesn't really matter what is being said. The important thing is that you spend time on the phone together.

If you live close to eachother, then ask if he wants to go out to get a coffee or something. Take a walk in a park, whatever. Keeping a conversation going face-to-face is much easier than doing it over the phone (you can talk about all the strange people at the place you're at etc.).