My grandmother has Alzheimers and when she was at her worst she refused to get in the shower, she said that she cleaned herself up and that was good enough. My grandmother lives with my dad and stepmom. My stepmom would tell my grandma to come and talk to her while she took a shower. When stepmom was done she told grandma it was her turn. Grandma would get in (she refused to use the shower chair) stepmom would hold her arm with the curtain closed. They have the routine down now to every other day. It is working well. You may just have to talk him through all of it and help him or tell him about the agency will send someone.
I care for my 89 year old father-in-law. At his sickest I had to give him a shower. He used the shower chair. At first he was very selfconcious about me seeing him naked. I told him that I wasn't paying any attention to him, that I was only there to make sure he didn't get hurt. After that he was fine with it.
Sorry for such a long reply, but I hope it helps.