This is going to come off as sexist, but it's human nature being that way--not me.

I have good news and bad news. The good news is it isn't your fault.

The bad news is it's his fault. Even worse, he doesn't know why and probably won't for a long time.

Obviously, he is still aroused by you. He wouldn't keep approaching you for sex if he wasn't. You're still attractive to him.

Unfortunately, you're losing attraction for him. Sweet, kind, good, nice.. these are all the traits you may use to describe him, but I can find these traits in my 80-year-old grandmother. He's not igniting passion in you. It's really up to him to figure it out, otherwise it won't seem genuine. And you can't really tell him he needs to to XYZ because you will know it's all a ruse.

Maybe start giving him compliments on the traits you like about him that turn you on. It will encourage him to improve those traits. Maybe he makes a cocky/funny comment every once in a while, stop and tell him it turns you on (if you're into that stuff). Maybe his ass looks good in those jeans. I don't know, you get the idea.