I'm sure that situation is very frustrating to you but I admire your devotion and willingness to stick it out with him, even if he's not sticking it... well, nvm.
Ok so I hope you didn't think that was in poor taste, and I'd like to help you out, as someone who's gone through this kind of nasty stuff before. First, your husband is at that point in his life where he needs to be looking at lab tests for hormone levels. I know he's busy, and I know you don't think he can fit it in, but he has to. There is so much to the chemistry of sexuality, but it is goverened by our hormones and neurotransmitters. Yes, there is the mental aspect, and I'll get to that. To start with, here are some important things to look at for any man:
Free Testosterone / Bio-available Testosterone
Estradiol (not just for females - we have it too! and too much is bad, as is too little)
Cortisol (salivary is preferable for capturing the natural rhythm of cortisol secretion)
The fact is, medications can alter these hormones, as can the aging process. When our hormones go out of balance, we may lose our ability to desire sex but we also begin to lose more as well. Neurotransmitter levels that are involved in sexuality, such as dopamine and norepinephrine, are also dependent upon hormone levels and may themselves be altered by the medications your husband is taking.
Basically, your husband needs to know if he has deficiencies. You never know: a simple test could reveal low testosterone, low thyroid status, or something else that you two may never have looked at.
Also, there is the mental aspect as well. Does he talk about his feelings? Do you know for sure that he feels adequate for you? Is there a lot of stress, or perhaps lingering depression? There could be mental elements holding him back too.
But you must, must get the physical side checked too! I say get the lab work done!
Hope I helped and hang in there