Mimi has a good suggestion. And I agree with her that showers are a real struggle. It is even harder when it a father I think. Luckily I have my son to help with my dad. Not that it is any easier however. Dad still has a mind of his own and can be difficult about some things.
Showers are easier for them. Do you have the flexible shower head and a bench that your dad can sit down on? One of the big problems is that showers are slippery. Also, grab bars are helpful...
Another thought would be to get outside help. There are agencies that will come in and, for elderly people, a weekly appointment would be enough. IF they can't get them in the shower, then a sponge bath is better than nothing. Plus, your dad may decide it is easier just to shower on his own than fight the outside help!
Anyhow, just a few thoughts here. I hope they help. Good luck!
Hugs from Glenda