I think you have every right to feel weird about this, it's something that you feel uncomfortable with and, whilst some people wouldn't see it as a problem others would not see it as normal behaviour.
However, remember that he's taken a huge step by telling you this, it must've taken a lot of trust and shows a great deal about your relationship that he can tell you something of this nature.
Be careful not to offend him in what you say - have you talked to him much about it or has he just told you what happened? Please talk to him about it, find out what he thinks about it, whether he's that bothered.
Remember it's in his past, I'm sure you know your boyfriend very well after being together for a year and love him for many reasons. This happened many years ago so I hope it doesn't have too much of an effect on how you feel for him.
Talk, talk, and talk even more to him though!