Preface: I (M 24) tend to have a high sex drive. However with my last gf it went insanely low despite her being attractive and all that jazz. The reason why mine dipped so low was for a few reasons.
1) It was so much fucking work. To get her off it needed to be half hour of sex, 40+ Minutes of oral, and I had to be dominant the entire time. It was so much work that I had to consider if I felt like putting in that much work.
2) She was awful at initiating. She would look at me and say "I'm horny" or she might just start grinding on my leg. Neither of those options were attractive.
3) She might extend sexual kinks outside the bedroom which I do not like. She was in to S&M and occasionally she would just say things like "and who owns you?" because thats what she wants to be told. But I don't like that outside the bedroom. So every time she brought that up I would just instantly shut down.
4) She would then talk about how she wants more sex. Which I understand. But I already fucking knew. She didn't have to tell me. I could sense it easily. So that meant that every time she started humping me I had to go "well I need to give her sex" which just made it more work for me!
Solutions:
1) back off for a bit. Don't ask him to go to the doctor or pressure him to do stuff like that. It just reinforces the idea that he is broken.
2) Initiate in better ways. Don't mention sex or that you are turned on. Just make out with him. If he stops then it stops. However it will make it so sex is not something he thinks about and is just something he desires and goes after in the heat of the moment.
3)Be flirty in ways that don't lead to sex. The point is to get his hormones going but don't make him think "oh this is going to lead to sex and how do I feel about that".
4) Try to take some stress off of him. If there are a few chores you could do then do them. Maybe one night when he gets home have dinner ready and a movie to watch. He won't be thinking about all the work he still has to do and will instead just be thinking about how he is able to relax.