Hi
I assume from your post that you only have a shower, not a bath? If this is the case, I agree with nitram, that gradually working your way up, may be an answer?
I have a shower over the bath (flexible hose type, thankfully). I used to be able to get my wife in the bath daily but around a year ago, she got up to go to the loo in the night, must have fainted and managed to break two bone in her foot!
The resulting "boot" she had to wear for a couple of months meant she couldn't bath so I got used to giving her a strip wash. Even without the boot now, she doesn't like getting in a shallow bath and washing her hair (which I try to do once a week) is a real mission!
I struggle to get her in to a shallow bath and even more to get her to actually sit down. Once sitting, she's fairly okay with me helping her wash and I then start the shower (unfortunately, my mixer valve is the type that's either on or off - you can only regulate the temp). I gradually work my way up her back and then to her head. At this point she usually tries to get out but a gentle elbow on her shoulder, plus a lot of "this won't take a minute" or "nearly done" and I manage to get it done - takes a long time though, and water everywhere!
Like nitram, I did try getting in there with her when she refused to sit down, but her protests, plus the shampoo suds making the bath very slippery, nearly had us both falling out of the bath! Amusing in retrospect but not at the time!
As jennypie has suggested, gentle persuasion and distraction can be effective ways of getting your husband to shower. As dementia sufferers don't like water on their head/face, you could try a shower cap? I was once advised to try swimming goggles but my OH wouldn't have it
Good luck
Phil