Starting to date again.

Long story short, my ex and I broke up in January and I've been trying to salvage the relationship since end of March, beginning of April (bad mistake) and I've decided to move on. I met this guy while I was doing my clinical rounds at the hospital and we exchanged numbers. We've hung out a few times and we text, but when he calls me, conversation starts of pretty good, but then turns into dead silence. (He also just got out of a long-term relationship) What are some topics that we can talk about so there's no awkward quietness?