I went through this exact same problem with my daughter, only she was younger, like in her twos, I think. It got to the point where I couldn't give her a bath unless I took a bath with her. (which probably won't work as well for you, seeing you have a grandson...)
Since I was still working out of the house at the time and couldn't deal with a big struggle, that's what we did, for about a year. I gradually was able to wean myself out of bathing with her, but it took a while. I just replaced baths with a shower--which we still take together, and she's going on 11. Yeah. Well, it's either that or she doesn't/won't take a bath. I figure there are other things to fight about/make a stand on. I also figure at some point--maybe when she hits her teens--she won't want to take a shower with her mom anymore. When we wash her hair--even in the shower--she has to use a handheld towel over her eyes, otherwise I can't rinse her hair (she doesn't like the water in her eyes).
My daughter also has Sensory Processing Disorder, which is a very complicated neurological issue, and I think that has a large part to do with her hating baths. Combing her hair when she was younger was such an ordeal--it hurts just putting a comb through her hair. Washing her hair in the shower can also be a big deal, if her hair has too many tangles in it and I pull her hair/make her head hurt. I usually make her comb her hair before we get in; that usually solves the problem.
When she was young and we took baths together, we also played with toys in the tub. Well, she played and I washed her--body and hair. Still took a while to take a bath, but I had a clean baby. Note that we usually only took 2-3 baths/week. I didn't have all the time in the world! When I was in the tub with her and washing her hair, I'd have her lean back against me, almost lying down but on top of me, and then I'd wash her hair--the water wouldn't get in her eyes that way. When she got a bit older/used to the water again, and I washed her hair in the tub, I gave her a hand-size bath towel (one of those smaller ones that get put out for people to wipe their hands on) to put over her eyes. It got soaked with water, but she at least let me wash her hair.
One thing we also tried that my daughter liked were the wax crayons that can be used in the tub. She could draw with them and then we'd wash off her pictures until next time. She liked that well enough that bathtime wasn't at least dreaded.
Does your grandson have any other issues other than bathtime? The bathtime incident with my daughter occurred at the same time as a variety of other issues, which we finally had confirmed as Sensory Processing Disorder (SPD) when she was 5. So you might want to remain alert and observant to any other issues that seem "over the top."