he doesn't feel safe

His compartmentalizing goes beyond the normal PDA reluctance. It sounds like something about your relationship represents something that he would never want his friends to know about. Sad to think about a kid out there somewhere going through this (his relationship with you) alone. But that's how it's got to be, right? You can't go enmeshing yourselves into each other's community and friendship circles, since the more people who see you two together the more likely it is that they will have run into someone who gossips about your crimes. Neither one of you wants to deal with that. It's hard enough on him just knowing that you are a sex offender, imagine the stress of having to defend his relationship with you to his peers and their parents.