Petunia - you sound amazingly grounded, and from what you've written, it seems like you and you SO have decent communication.
People have made excellent points. One for your SO to remember is that all relationships change continually. I think some folks just fear change, and overlook that it's already going on all around them. It's better to face it and try to influence the outcome positively. He needs to think deeply about what his fears are and create strategies for overcoming or at least managing them. "Being less cool" is often code for getting older, being more stable, not having fulfilled his desires yet for career, hobby, not being content with appearance, etc. In the end, we may all leave relationships for a variety of reasons. No amount of worrying is going to change that.
"Love takes off masks that we fear we cannot live without and know we cannot live within."James Baldwin