Using gay as an insult? Could use some advice.

I'm not entirely sure how to start this post off, but here goes I suppose. I've had this friend (who shall rename nameless) since I've been in fifth grade. I never really had a moment where I came out to him (or not that I remember), but it's pretty apparent now and he knows. We've both changed quite a bit over the years (as one would going through high school) and for the most part he's been pretty accepting of me being gay.What you should know about my friend is that he can be somewhat stubborn and judgmental. Despite being judgmental he isn't really racist or homophobic or anything like that but instead has little to no filter when it comes to certain things. He's got a somewhat dark sense of humor but usually tries to play it off as just goofing around.Up until now I've been pretty tolerant of his dark sense of humor and such but lately I've noticed he's been using 'gay' as an insult for nearly everything. So recently I addressed him about it and asked him politely if he would avoid using it, or at least around me. He got pretty upset that I was trying to 'censor' him and wouldn't talk to me for a few days. Due to him being my best friend, I apologized a few days later and he allowed me to further explain why I didn't like him using the word like he did. He simply responded "I guess I can see where you're coming from." but didn't really seem to consider to stop using the term or anything like that.This is my best friend, and I'm really not sure what to tell him or how to go about the situation now. I've been letting it slide as I suppose I can deal with it when it's not directed at another person. However it's got me questioning how great of a friend he is if he's not willing to stop using it the way he does?I'd also like to further clarify that I'm fairly sure he's not homophobic, as he's told me he's attracted to trans women before in which I assured him I was just as fine with as he was with me being gay.So my question is, how should I go about this? Should I try talking to him about it again? What's your opinion on the word gay being used as an insult?