Wow how exciting for you all! I have traveled only with a 4 YO girl, so some things will be the same, some not so much

Are you taking the whole immersion & using the dining plan? if you are, start looking at restaurants. IMO, you should pick your parks first, then stick with restaurants in the park. for a four day trip with a 4 almost 5 YO I would not traipse them back and forth a lot -- everything takes twice as long at that age. (I think that 5 is THE worst age)
For sure Hollywood studio's will be a hit with all of the Pixar characters, the stunt shows, and the Toy Story Mania. Give your little man time to play in the Honey I shrunk the kids playground -- work it in between a show or two, then he can burn some energy after having to sit still.
I personally don't find the food all that worthwhile, but my DH and my DD really got a kick out of eating in a car and watching campy movie clips at the Sci Fi Dine in. We all enjoyed the character meal/buffet at Hollywood & Vine, although when we went, JoJo was still on TV -- I read that Handy Manny makes an appearance now as well as the Little Einsteins. If these are chars your grandson likes, I would pick this....if he if a car/tv dude, go for SciFi Dine In.
At Magic Kingdom don't miss tomorrow land & Frontier Land. The BuzzLightYear ride and stitch attraction are terrific, and frontier land is lots to do for a 4 / 5yo.
Epcot's don't miss will be Test Track, Soarin' and Turtle Talk with Crush.
Animal Kingdom Khali River Rapids & DinoLand
I think these are the absolute top hits ...there is SOOOO Much to do and see
Do have a plan -- even if its not a hard and fast plan, go on the Disney Site and take a look at the different rides and attractions and mark as favorites the ones you will most enjoy. make a list & either read it over in the morning or take it with you each day. Its sooo easy to get overwhelmed when you get there that having an idea of the things to see/do each day really really helps.
I look forward to seeing you around the boards!
