How about you see the cult first hand. Do some preliminary research, propose to your boyfriend to accompany him, attend that convention. See with your own eyes how it works. Watch the manipulative soap operas they use to control the sheep through guilt trips at every turn. You may even like it, or maybe not.
If your boyfriend refuses to take you to the convention, then that could mean he is embarrassed by the religion or he actually is hiding that he is still mentally in. Either way, knowing all the facts about your "enemy" will help you deal with it in the best possible way. Keep your friends close and your enemies closer. Determine where you boyfriend is, with you or with them? Sorry to put it so harshly, but that's how cult members are. It's all black and white. We versus them. Either with Satan or with God. No gray areas.