Glad to be of help, I hope you guys soon be able to resolve and overcome your issues and enjoy all the love and intimacy you deserve :)

To me it seems like he likes the idea and wants it "in theory" but may not be his interest in reality or is just confused about it altogether (which explains him trying & then backing off when he was really in the situation) so he's either consciously/ unconsciously avoiding it till the time he understands himself and his thoughts on the subject or maybe really waiting for some "right feeling" to do the deed (which is a slimmer possibility)

Honestly it looks like it has more to do with himself than you or the relationship. He values you & likes you and loves you for real but he's unsure of certain things in his mind

Shown Interest as in expressed it romantically & sexually? & Then it decreased or ceased?

Also, Is this his first relationship? And yours first too?

Could he be asexual? Or aroace (that is both asexual & aromantic)... The relationship with him does sound like that of two old friends who are very comfortable with each other