This reminds me of my fiance's roommate a bit. This happened like last year or so, and this guy was apparently upset that he wasn't in the middle of our relationship, like how me and my fiance would make plans together or just stay home and chill together, he wanted to be part of everything we did basically, and I think it was because he was jealous of me and wanting to spend more time with my fiance, not me. It was super weird because when he started doing this we had only known him for like less than 6 months. He's not bi or gay or anything, so it's not like it was a romantic attraction I just think that he enjoyed hanging out with my fiancee so much and thought he was more deserving of his time than I was, like he was the better option to hang out with.

Anyway, I'll tell you what we did we backed off from his friendship because obviously he was seeing something in the friendship we didn't. We still talked to him and stuff occasionally but no where near to what we used to do. A year later now we are at a healthy medium with him, we'll occasionally hang out and game but he dosen't get clingy with us anymore, asking to hang out in everything. You could try that OR you could try to confront him, but it sounds like he's not even responding so you may just need to back off a bit from the friendship.

I think what happens to people, is that they don't know how to react to their friends being in relationships and they see the emotional intimacy that's in the relationship and they want that with their friend too, and so they get jealous. He could be romantically interested in you or not, but he's for sure jealous of something in your relationship.