If he's into Potter, and wants to ride the train (which is a great way to enjoy a ride), you need park to park tickets. Get there early, and try to go to the Studios first. I find they handle crowds better in the Potter area than in the Islands. So, do all you want in the Studios, then take the train and work your way from Potter to the front of the Islands park. You won't be able to do everything, but check out what your grandson likes the most, and then make sure you do that, and anything else will be extra. The make up show and animal actors show, both in the Studios, are great shows to see. I'd try to go to both, and rest the feet. Both are not long shows.
What a nice treat for your grandson. Unfortunately, I haven't known any real discount for a one day ticket. Even a park to park one. And if you park in one of the massive garages, give yourself a good 15 minutes to get from parking to the park. Security lines can be long, and the walk, even with the moving sidewalk, is still quite a hike, depending on where you park.
If he wants to do the water rides, all in Islands, make sure you rent a locker, and have a change of clothes, or put him in a bathing suit, and easy dry top, and wet friendly shoes. You can get really wet in all three of the rides, with the dinosaur one, the best chance of not getting as wet. The other two, you might as well may look like you jumped in a pool.