In my experience you just have to do whatever works. If he messes in his underwear then you have to tell him you must clean his back/bottom, whatever word works, then do your best to clean him as thoroughly as possible. Think of him as an infant: not able to care for himself. You have to give him drinks of water, offer food every 4 hrs, whatever you think is reasonable. I tell myself to treat him, my mate and husband of 50 yrs, as I would like to be treated. This always puts me in the right mind.
That was a problem for me as well. I think part of it gets to be that they don't always know who you are and don't want to undress in front of you. Luckily when he had an incontinent episode he then would shower and usually let me help him after some coaching. Patience is key - try not to get angry. Pick a good time of his day - not during sun downing! A picture chart of what is going to happen like undressing, showering, toweling off, redressing -may help. At first just setting the water temperature in the shower got him in there. Try to analyze what the reluctance is: unable to know how to start, not understanding directions so does not know what he is to do, doesn't remember where the shower is or what it is, doesn't know you, etc. Good luck.
We're always trying to get water around here ! He's been to ER from passing out. He always tells me he can't drink so much water. Two for a day & nite all together. Someone know anyway to do better. I flavor it. I mix it with lemon ade & give him choc milk. Everything I can think of.