You've done the good and honest thing telling him the truth, the best thing now is to pray that he will take it well, even though TRUSt in any relationship is something one has to EARN. Of course this is easier said than done!!!!!!!! I'll be praying for you though.
This story might encourage you a bit, before the start of my current 3 year relationship I had been out for about 6 months with this other guy ( who I will call X) in church that my boyfriend also knew, they were kind of acquaintances, only on a a 'hi' and 'bye' level, not really close friends at all. After I broke up with this guy and started my current relationship my boyfriend asked me if I had ever gone out with X, as it seemed like we were always talking to each other in church etc. and seemed pretty close. As a matter of policy my boyfriend had always been totally honest adn open about past relationships. I of course fearing the worst, lied about this and said know we were just good friends, because I feared that my boyfriend may not want to start a relationship with me because I had previously been out wiht X (don't know why i though that).
So basically I lied and I kept this past relationship secret from my boyfriend for about 3 months I think. Eventually X, who had not really got over our break up started to notice the relationship between me and my boyf in Church (which only started very shortly after X and I broke up), and he began to confront me about this and even threatened to confront my boyfriend. It was a this point that I thought rather than my boyfriend find out from X about my relationship with X in a scenario that could be potentially hurting to all parties, I finally summed up the courage, amidst much crying, told my boyfriend about my past relationship with X, why I had lied about it and kept it a secret from him. i ofcourse expected my boyfriend to be totally devastated about my dishonesty, but you know as God would have it, he only loved me more for it, and was glad I had told him. Infact X eventually confronted him, but ofcourse my boyf already new about it, so he handled it well, and I think X was really shocked that I had told him about this. Now my boyfriend and I have a stronger relationship than ever, and it only showed me more than ever that although the truth hurts, the long-term results of honesty are certainly worth the initial pain. The longer you put telling the 'secret' or the 'truth' off, the bigger the elephant is fed, eventually one day you'll crumble under the fat elephant's weight!!!!!!!!!!!!