My bf and I don't have the same sex drive

ok so my bf and I (both19) have been together for 7months. at first we would have sex all the time, and that was great. actually he was the first man to ever fuck me and consciously or not he turned me into a power bottom haha. the thing is that lately I feel like whenever we have sex I have to specifically ask for hit.he's said it many times before, if u want dick just say it, yet at the same time I miss those times when he would just initiate it or follow my moves when we were at bed, like he was actually into me. now that our relationship has gotten more serious, I feel like he just lost the interest on me.honestly I think he just got bored. he's so fluid and he's been in relationships before where he was the bottom, so he has proposed the idea of going on a threesome bc he misses getting fucked. the whole idea of the threesome freaks me out. like I also want to try threesomes but with two people that I don't know, I'm not comfortable with the idea of just letting someone else in our bed. maybe I'm just insecure because of the fact that in every trio there is a duo and a solo, and I'm afraid to be the solo most of the time given my 'little' experience.I have proposed several times to fuck him but he's not into the idea either ! I just don't know what to do. Since its always me the one to initiate it, Ive told him that next time he actually wants to fuck me he should let me know, cause id rather have sex when he actually wants it than when he's not so into it... idk what to do I have this very high sex drive and I don't even know what is it that I've done to make him loose interest on me. help