I think it is tad unfair to think that all men have high sex drives (implied by some of the posts. . .or rather I inferred it). This is like the cliche that all women don't enjoy sex and are just 'doing men a favor' by having sex with them.
With that said. . .I had a similar problem with my boyfriend. Stress and some low self esteem issues (which were not obvious in any other parts of his demeanor or behavior) had a huge dampening on his sex drive.
Advice:
1) Yes, he probably should talk to his doctor to make sure he is physically ok. Don't pressure him to do this. If he is going for some other reason, perhaps suggest that he get an overall physical to make sure he is perfectly healthy.
2) Sounds like he fulfills you emotionally and in other aspects of your life. Ask him if including more sensual, rather than sexual, experiences could be integrated into your lives. What do I mean about that? Massages, regular touching during regular/boring daily activities, exploring each other with touch (with an agreement no sex is intended). It may not be sex, but feeling connected physically will help you feel the intimacy, and he will feel less pressure to perform (particularly if he is having issues with becoming aroused and is too embarrased to discuss as of yet).
3) Patience. Don't nag, but telling him things like you find him attractive, leaving him sexy notes, etc can slowly perk his interest back up again. . .but don't do that every day. Try to think how you would feel if your sex drive was low and how you would want him to approach you.
Good luck!!
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