I’m not sure if my advice is helpful, as my son is a bit younger, and so more of a child then yours which is more of an adult…..
however , my DS 14 would quite happily never shower or brush teeth .
the only time he ever washed without kicking up a fuss was Sunday evening, as since a baby Sunday nights every one showers and hair washes ready for school Monday ( a left over routine from my childhood, and even further back my mums lol)
the arguments were horrible and so was the BO !
we installed a new routine (rather then random days , so he knew exactly what was expected/ what he has to do )
every Tuesday, Thursday and Sunday he had to shower , none negotiable. Teeth brushed every night. Any carring on and he immediately lost privileges, no moaning , no whinging, no trying to get out of it or forgetting.
after three weeks of resistance, he now follows that routine without any prompting, and has done since September.
nb I do think he’s possibly on the spectrum, and the routine of knowing when exactly he HAD to shower helped him
nnb before anyone says im mean , he can of course shower any time/ day he wants extra , I just ask he lets me sit down first so I don’t faint of shock 😳
perhaps that’s something you can do? Say you’re not willing to put up with the smell , and it’s imperative that he showers on these days , none negotiable, but if he’s willing to accept that with no reminders , perhaps you could be persuaded to buy him - insert small bribe - to make it not such a shock
hopefully that gave you some ideas x