You did the right thing of not forcing him to have sex, and you should make it clear to him so he doesn't feel obliged or guilted to have sex, that it is not his fault that he is asexual.

It's understandable that he is coming out with 'solutions' to make things work despite your conflicting sexuality. Unfortunately, you are incompatible with each other. It's no fault of either of you, as neither of you can control your sexuality.

You need to explain to him that sex to you is an intimate thing, both emotionally and physically, and you are not willing to have sex with someone else when you are romantically involved with him.

(If you are involved in fandom subculture, you can explain that this is like watching your OTP being intimate with someone else. It's uncomfortable, to say the least.)

There might be some ways to make this relationship work, but ultimately you might need to decide whether it is worth it to continue with this relationship with no sex.