How do you help/encourage a man to solve an erectile dysfunction problem without pressuring him (which makes it worse)?

Posting here because RPW doesn't accept random first time posters (for good reason).

ED is ruining an otherwise perfect relationship. We went from being all over each other, him being the man in charge and me being the woman following, to being basically asexual BFFs over the course of three years.

He is fine with this arrangement, but I'm not.

Still, I can't just leave the only man I've ever been with and ever loved. I can't imagine ever having sex with anyone else or loving anyone else, so I would be just as repressed (as well as depressed) if I did leave him.

Just ending the relationship would seriously hurt him emotionally, as well, and I could not live with that guilt, either, even if I felt nothing else for him--and because I do love him intensely, that love combined with extreme guilt, makes it impossible to leave (and I'm okay with that because don't want to leave). So "just break up" wouldn't be helpful advice for this situation.

Possible Causes for his ED:

-Age. He is 37. I am 23.

-Low testosterone. He is not interested in getting it checked and does not feel this is important. Still, he works out, so this might not be the problem.

-Boredom from long term committed relationship of three years. Men need variety.

Definite Causes:

-Pain and exhaustion from working out. He doesn't work out everyday, but on rest days he still is tired from the workout days.

-Lack of interest in sex. He does not initiate nor feel the need to.

Not a Cause:

-Porn. He does not watch or enjoy porn, thinks it is stupid.

-Me getting fat. I have not gained any weight during the relationship, or changed my body in anyway. I look the same, dress the same, same hair length, etc.

-Him getting fat. He is not fat at all.

-Him cheating. He is religious and not the type to cheat or to have sex with more than one woman at the same time (no plates, just committed LTRs).

Already Tried:

-Acting sensual (stripping for him, touching him with hands and mouth). He enjoys this mentally but has no physical reaction.

-Not mentioning sex at all for weeks and seeing what he does. Nothing happened. :(

-Asking for sex. Sometimes he says no, other times he says yes but then can't get it up.

So what do I do?

TLDR: During the course of a three year relationship, the man went from wanting sex twice a day to not wanting sex at all, and this is causing problems for the woman.