Yeah, I just don't understand it because why wouldn't he want to feel clean and nice? I know if I can't shower for whatever reason, and say it's been hot, I feel gross until I shower - it's a horrible feeling, and you feel so much better once you've showered. A bit like if you've been unwell, and then you finally get to shower and sleep in a clean bed - you feel so much better. He must feel grim. I even get enjoyment out of picking new little shower gels and cremes, different fragrances, it's nice. And how can he want to kiss someone when he knows his breath smells?
Surely he must feel different when really clean, freshly showered, nice teeth, in decent freshly washed clothes to go into town for a drink vs in minging clothes, no shower for a week etc. It just seems weird there's nothing behind this.
I'd sit down and talk to him properly - or write a letter. Explain how it impacts on you, that it makes you feel unclean and sad. Don't attack him but make it very clear in simple language that it is really unpleasant.
I'd also try to spruce up the bathroom, buy a new flannel (you can get them for 50p so not going to break the bank), one of those plastic body scrubbers, some nice shower gel for men, new towels. Yes I know you shouldn't have to but at this point I'd be desperate to break the pattern and if he doesn't use them you can. Maybe he could get his hair cut. I think a decent goal would be simply to shower once a day - if you start getting into every other day or missing days then he may end up missing several in a row and there's more scope for things to go wrong. If it's simply a quick shower once per day it would make a world of difference.
Is he almost being defiant because he's embarrassed and is like well I'm not going to change? So digging his heels in?
I'd focus on the positives and explain you WANT to be attracted to him, you want him to smell nice and be clean etc and what a difference it would make.
It's fascinating really, it's such a simple thing he is neglecting... for what?