When our Tippi was getting old and ill, she couldn't wash herself, so I gave her baths. I won't say she loved them, she was a cat and they hate water, but she was surprisingly cooperative. Perhaps she couldn't stand her own smell!
What I did was, dissolve the shampoo (a tiny amount of washing up liquid) in about four inches of warm water in the bath and gently lift her in. I think the warmth helped ease the pain in her stomach. When I thought she was as clean as I could get her without the water getting too cold, I lifted her out, wrapped her in a towel and gave her a good cuddle. That bit she did love. When young and well Tippi was a very spiky miss, and gave us several scratches, some nasty, but she didn't even try to scratch me when I bathed her. She died twelve years ago (thyroid and suspected cancer in her gut), and we still miss her.
Do please stop any fish. We've kept the fish in our cats diets to the minimum. One of ours developed ezcema through fish in his food, while another cat we knew developed epilepsy and died in a severe fit.

Good luck with your grumpy old man.