Is he sexually attracted to other women, or is the whole sex thing in general just no longer an interest? Has he been tested for fatigue factors or testosterone? The later can have a big effect on men, and there are medications for that. Lots of physical (mental/emotional/spiritual) imbalances can have that effect. If he's willing to get checked he should, but that's one of the symptoms - they don't care to.
You say you're best friends other than this. My wife and I got married the other way around. We knew each other thru several relationships (I had girlfriends she had boyfriends) and it was the fact that we remained best friends even as these more passionate relationships came and went that got us to the altar. It's funny, our situation is still kind of the other way around. I try to touch her and she laughs and pushes me away. It's an uncontrollable tickle/giggle thing, or so she says. And it started when she went thru the change. She says she wants sex, but I try and get rejected by this laugh. It's not disrespectful, it's just ... in the way. And she has been checked by her Dr.
Back to the point, it's a best friend relationship vs ... well, what are your options? You gonna cheat, buy a toy, secretly slip him viagra? We both know what God says about that. But I understand the predicament you're in. Trapped in a flesh full of organs, hormones, and neural pathways determined to make us want to 'become one' with another. How do you make the twain meet?
Lord God, look upon Alley, I come before You with her to seek Your will and discernment. Thank You Lord that by the Blood of Your Son You allow us to, that is so awesome. And I know that even before we approach, You see, and care, and have an answer to the situation. And so still our souls, Lord, that we may hear Your words and discern Your way. Lord God I lift up this husband, this church going man who refuses his spousal duties. Bring him words of wisdom, and knowledge, that he might understand; bring him healing in whatever fallen flesh affliction enslaves him. And bring Alley temperance; we know that in Your Spirit the flesh should not inflame, and so may Your Spirit flow like water to quench the fire - not to extinguish, but to lessen the fleshly handles the tempters so like to to tug and pull upon. Lord You created sex, and it has a place withing marriage; I ask You that it's rightful place be restored among all of us - and among the marriage of Alley, as she has faithfully sought after here. Thank you lord, for allowing us to seek You, praise You Lord for Your answering grace. In Jesus Name we lay this before You. Amen
God bless you.