A bit late, but I am in a similar situation to yours.
In the beginning of our relationship my boyfriend and I had an exciting sex life, which I think is largely common at first in sexual relationships in general. In the beginning we'd be having sex 3 times in a row and never thought a thing of it- and now if we did once longer than 10 minutes it would be an achievement, as within the past year it had dwindled into a series of mostly sub-par sporadic encounters.
I have talked to him about it and discussed various medical/psychological reasons why this might be happening and asked him to be upfront and honest about it, and it surprised me to learn he has some self-esteem issues over weight and performance of his own, which cause him to be kind of hesitant and disinterested now, in a way avoiding having sex because that is easier than all the baggage and thoughts that comes with actually doing it. In the beginning of a relationship you are more likely to just dive in and go after the "wants" that you have, when after you've been with someone for a while, they start to voice their opinions more- telling you what they like and what they don't- then you are always thinking about if they are liking what you're doing.. what you look like.. etc. so I could totally get what he was talking about.. but he admitted it being more about his body image issues and laziness than an actual disorder.
I think you should just put it on the table that there is a problem, ask him his thoughts on it- does he feel like its something he really can't help? or is he feeling like hes pressured to have this big sex drive/being self conscious? The sexual part of your relationship is an important one. If he's feeling awkward about it, it might be easier to discuss it with you than in front of a doctor. If he has a hard time talking about his feelings, get him to write them down and just read it. Maybe he is a classic case of this HSDD.. and if he recognizes the problem is beyond these home-style remedies then the best thing very well could be talking to a professional.
Good luck with this, you obviously care about him very much, and that can only help matters.