Boyfriend anxiety

So my first boyfriend has had 3 girlfriends before me, his last ex being the most serious relationship. So far we've only made out but I kinda want to know how far he's been with his previous girlfriends, just to know what to expect and if he's comfortable with going farther. I feel like it would be weird if I ask him and I kinda questioned our mutual friends but no one really knew either. What should I do?