My boyfriend has gotten off by just tit fucking me. We both really enjoy it. As long as he's not putting his weight down on your chest, you don't have to worry about breaking anything. He's thrusting between your boobs toward your face, generally; the force isn't directed down, towards your ribs.
Also, don't worry, you're not going to pop your boobs (unless you have implants; I have no experience with those). We've been pretty rough on mine, and pushed them together pretty darn hard. They've never shown any sign of popping. I've also seen some pretty extreme tit bondage. Seriously, go look some up. It's reassuring. If boobs can withstand that kind of heavy constriction and beating, there is no way tit fucking and pressing them together is going to pop them.