Hi Donna,
Your grandson is going to love Disney World! Most of the attractions at Magic Kingdom are well suited for young children, so I would start there. Take a look at the
Attractions page on Disney's website. The ones that say Guests "of any height" are the ones for your little guy.
I suggest you look at Disney's
Planning for Pre-School Kids & Little Ones page. It will help you plan a kid friendly vacation for you and your family.
One more thing, Disney theme parks have
Baby Care Centers. You can take a nice break from the crowds, and have quiet feeding and changing times. There are baby supplies there that are available for purchase if you find yourselves in need of something for your grandson. They carry disposable diapers, a few different brands of baby foods and formulas, as well as some over the counter medications.
I hope this information helps you. Wishing you a very pixie dusted trip with your little Prince Charming!