I just don't know what to do. I would like him to be accepting and allowing to open our relationship, but I don't want to force him into anything he doesn't want to do. I'm really stuck between a rock and a hard place.
It may feel complicated, but really it is not. What is YOUR time limit? Can you wait a year? 5 years? Life? You have to have a serious talk with BF. Don't mind reader or guess -- just have a serious talk and take his temperature on his wants, needs, and limits. Digest the new information and see how it lines up with YOUR wants, needs, and limits and where the common ground lies.
If he is NEVER ready, can you live ok with that and be happy in your relationship with him and build life with him? Just as it is, today? If so, then stop with messy boundaries with the friend
-- keep him in the friend bucket, do not encourage talk about him becoming a secondary, stop sharing sex with him. You end up confusing yourself. And stop with the messy boundaries with the BF
-- no jokes about getting together with the friend to sex it up or yank your chain. It is weird to me that he would do something "only for you" -- what? He had ZERO desire? Why is he compromising himself?
If you want to explore your relationship with the friend, and BF is not open to a polyship and says he NEVER will be then you have to break up with BF. It is not fair to string either of you along if he's not happy one way, and you are not happy another way just to CPR an ending rship along a bit longer. Set each other free, mourn, and then start over so you can each find your happiness. Try to be decent exes to each other.
If it is something in between those two extremes -- then ask BF if he is willing to Open just in Mind and explore some resources like this forum and books with you to try it on for size in the safety of your own heads. Nobody doing anything untoward, nobody dating other people. Just learning things and trying it on for size in your heads in a calm, relaxed, no hurry fashion. How did you feel about that article? What would you do in this situation like that post today on the forum?
Then assess where you are at in a year of study. It can take years for a couple to prepare well -- and however long you take that feels right to YOU two is right for YOU two.