I'm new here - and identify as mono to boot, so maybe this opinion is not welcome but I wanted to offer a different point of view on his use of possessive language.
Possessive language is so ingrained in our culture that for many people, it is often the goto way to express other feelings. Esp if this was said in the heat of an argument. When intense feeling of jealously, insecurity or abandonment are involved, it's easy to pick the words that come to mind quickly, ones we know from tv, movies or books, and they may not tell the whole story.
He may not mean "as long as your mine..." in terms of dictating your actions or beliefs, he may mean "as long as I stay your primary" or a host of other things. I can't tell your for sure if he wants to possess you without meeting him first-hand, but talk with him about what he really means by that before jumping to any conclusions.
Why is this my opinion? Because I've used this language before - usually while distressed and always without thinking first.