Asexuality is actually rather broad. But my definition would be someone that is not interested in having sex with people. It then becomes possible that someone experiences all other manners of attraction: Emotional, physical, aesthetic but not sexual desire.
Which creates the problem: The asexual person can love someone but not want to have sex or need to have sex. They can understand that sex is an expression of love like the majority of sex, but some will feel that it's not really connected at all and they don't need to have sex. There is a whole society of asexual with marriage problems in regards to sexual/asexual pairings.
For Sexual Partners, Friends and Allies
The thing with asexuality is that it's extremely fluid. Some of the members are attracted to the idea of having sex, imagine having sex, view porn, masturbate but when it comes to the actual act itself with another person. They won't be interested.
Hopefully this might solve the problem. It's just something I thought when you talked about low interest in sex, but it sounds like he loves you too. Else you wouldn't be together after 6 Years. Unfortuantely I don't think there is a cure as such, most of these couples have had to either compromise like this
thread.
Ask if he identifies with these people?