It sounds as though you have a great relationship, apart from this problem. Don’t let it ruin the relationship.
The first thing to think about is why his sex drive changed. There might be some medical reason, or a psychological one - which I think you might be hinting at.
If that doesn’t work, I think you should have some sensitive discussions about how, despite all your love, you still have some physical needs. @NastyRigPig gives good advice. Since you’ve negotiated some outside sex before, you could probably agree on some rules.
What I’m about to say next is the controversial part. If none of this works, cheat on him. It’s your body. Don’t let him control you. You say that your fuck buddy is in love with you too. I’d steer clear of him, because you’re risking ruining both relationships. Be careful, but seek out anon sex whenever you need it.