To all the people who say "He'll obviously cheat on you." just ignore them. When he was single he may have slept around, as most men do. They're not all going to say "No, I'm waiting for the one."
Before I met my girlfriend I had slept around a bit which did worry my gf a little bit because I was her only one. After a messy (short-term) break up we both slept around a bit but at the end of the day we loved each other so we put it in the past. If you can't put someone's sexual history in the past, then it probably won't work out. But to be honest, it's only a short term problem and eventually you'll get over it. Yer it's awkward because you know the people he slept with, but would you rather you know who he slept with than not? If he's been loyal to you for a year then obviously he cares about you a lot and more than likely loves you, how would you feel if he said to you "I can't be with you because you used to be a whore before we went out."
Just give it some time, talk to him about it tbh. Just tell him your uncomfortable about it but get it out your system mostly. It hurts to ask, but if you want to know something just ask anyway and be told, you'd rather put up with the pain for a few seconds than have a prolonged worry in the back of your mind. If you still can't after a few months maybe, well then I think personally you're being a little immature on how you see relationships.