He might just find it hard to trust people because of things that have happened in the past (not necessarily because of relationships with his g/f's, but with friends, family, etc)... that's my problem.
I think for me, I can only trust people after I'm 100% TOTALLY SURE they trust me. I know it sounds werid, but I figure if they can open up to and trust me, then I feel better and more comfortable about trusting them. (It also means if they betray me, I've got something on them). Have you shown you b/f that you trust him? If not, maybe that'll help.