If your significant other is uncomfortable with introducing you to ‘friends’ of his, that ought to be very disconcerting to you, whether they are platonic or romantic exes. It would be for me.
Without meeting this person that he values, this will always bother you. I would recommend that you assess your boundaries, and be brave enough to let know him what they are without ultimatums, and be brave enough to walk away with dignity. Don’t fall into the emotional trap of becoming a private investigator within your own relationship. Your dignity has value, if he has value, he will recognize yours.
Though I run this site, it is not mine. It's ours. It's not about me. It's about us. Your stories and your wisdom are just as meaningful as mine. 