Hi, Lorrie! Bear in mind that you are talking to a mom who took her kids to Disney World as soon as I thought their eyes could focus on that beautiful Cinderella Castle, but if this is going to be a once in a lifetime, magical memory filled trip, then I do have a recommendation for you!
My brother is not as big a Disney World fan as I am. He loves going, but if you leave planning to him, he's going fishing. As a result, his boys haven't been to Disney World as often as my girls, so for them, each trip is a big, memorable trip. The thing that worked best for him and his family was to go once his youngest son was tall enough for some of the "bigger" attractions. His first trip was with my oldest nephew, who was five at the time, and he wasn't tall enough to do some of the things he most wanted to do, which resulted in a couple of disappointing days for him.
Disney Mom Erin gave some great advice regarding height requirements in her answer to a question located
HERE, so you can get a good idea of how many attractions your grandson will be able to do of the things that do have a height requirement. I would say by 44 inches, he can do most of the attractions, and you should have minimal disappointment (although some of the bigger thrill attractions require 48 inches, you may want to wait until he's older for those anyway!).
So rather than wait until he is a certain age, get out the tape measure! By the time he is five or six, he'll have wonderful memories of a fabulous Walt Disney World vacation with his favorite grandparents!