You need to have a serious talk with this poor guy and tell him exactly how things are. Be aware that he might stop being friends with you after you set him straight, but to allow things to continue as they are is not fair to either one of you. He might continue to pine away for you and act jealous and be heart broken over and over and over, if you don’t set him straight. That is a very un-balanced and un-equal friendship to maintain and not sustainable over the long run.
You need to sit him down and say something like, “Rex, you know I love you and consider you to be my best friend. But we are only friends, in the platonic way, and I don’t feel any romantic inclination towards you and I never will. I’m sorry. You can’t force someone to feel something that they don’t. You can’t make people fall in love with anyone in particular. Sometimes the way you act toward me is painful and embarrassing for me, because I feel like I have to hide my feelings for other guys that I am interested in, so that you won’t be jealous and hurt. I don’t want to have live my life that way, where I have to keep my feelings inside. So you need to decide whether you want to be a real friend, and stop giving me grief and acting jealous toward me and stop expecting that someday I will become your girlfriend, because that’s just not going to happen. I value your friendship immensely, but I need you to be a friend and not a boyfriend. What do you think?”
Be prepared that he might be angry or embarrassed or hurt. But you have to set him straight, right now, or this situation will continue the way it has been going.
Also be prepared that he might say something like this, “Am I so hideous to you that you can’t imagine ever loving me?” or “We’ve been through so much together and I’ve always supported everything you’ve ever done. Why can’t you do the same for me?” or “Why can’t you just see that we were meant for each other, isn’t falling in love with your best friend what women always say they want?” or “Just tell me what to do, If you want me to be better, just tell me how I can do that?”
You need to have some answers ready that will still let this poor fellow know, in no un-certain terms, that you don’t have any romantic feelings for him and never will. But be as kind and gentle as you possibly can, even though he might yell at you (out of pain and embarrassment). Just don’t leave him with some type of optimistic idea that you might change your mind in the future.
And be prepared that he might have to walk away from your friendship, because staying would be too painful for him. Don’t be mad at him, if he chooses this route.