It will eventually come out some way. If it has no much bearing on how you are relating, you have no obligation to tell him. Your boyfriend should perhaps say it himself but make sure it does not bring bad blood between them. Past relationships can hold us from progress if we dwelt on them so much.
I would say tell him. I was in a similar situation once, this guy I used to go out with became my bf's friend (this was before I started dating my bf). No one knew about me and this guy And naturally I thought my bf would never find out. He never did but I always had this guilt. Once too much time passes you can't exactly tell your bf. The sooner the better. He will understand, it's nothing big.
It's an uncomfortable situation for your boyfriend. He wants to be with you, but he also does not want to lose his friend. That is one of the downfalls of dating after your best friend. Give your boyfriend some time to get his nerve up. If he doesn't it will come out sooner or later and will have to be dealt with.