This is only my experience, so take what follows for what it's worth (not much).

I have been in the same situation as your partner (though I had sex with my partner more ofte, but still way less than before medication).

Simply put, I was ashamed of my low libido and not in a good headspace even with my medication( which was supposed to help).

I was so ashamed that I didn't want to initiate sex with my partner. It was not rational, but that is how I felt at the time. And I would sometime watch porn so that I could get the dopamine I would get from sex, without the shame.

I know it won't make you feel better, but hopefully you can have a different perspective on how your partner might feel and get, hopefully, a new track to find a solution to your problems.

Also, there are some medications that can help with libido (Wellbutrin if I remember correctly). Might be worth it for your partner to check with his doctor.