Hi,
A new grandson! How wonderful! Congratulations! I can see he is one lucky baby to have such a thoughtful Grandpa! I think once a baby can really smile anytime after that can be "the best vacation ever!". The most important thing to remember is to take lots of pictures of him doing the things that please him most. While a baby won't remember the trip, he can memorize it through the pictures you take. We put photos in a fabric soft-sided baby approved photo album (which I bought in our local BabiesRUs) and let our girls keep it in their toy box. As they grew it amazed us how often they picked it out of the toybox and asked to hear their "stories" about the pictures. Your grandson will know what he did on his vacation if you tell him all about it as he grows. This really extends the magic for me and makes me so thankful I didn't wait a minute longer.
One thing I have noticed is that every trip seems to take on a life of its own, as my girls have gotten older. When my girls were really little we tended to make sure we saw the parade everyday. We did character breakfasts and dinners. We went on Dumbo 5 times. The carousel 5 times. They loved the repetition. You can always take them again when they are older...but you can never take them again when they were younger.
Let him take the lead, he will let you know when he is getting tired or has had enough for the day. If you follow his cues you all can still do heaps of other things after taking a break back at your hotel or in one of the baby care centers. All of the parks have clean, friendly and air conditioned baby care centers that are equipped with highchairs, changing tables, child sized toilets, rocking chairs. They also sell baby supplies, if needed.
On your trip, be sure to catch up with Mickey and Pooh. You could also start a new tradition by taking his picture in a favorite spot of yours. Every time you return you can take a new picture and see how he has grown.
I feel I should mention this one thing...there is a proportional relationship between child's age and amount of stuff you have to bring to Florida and carry around in the parks.

(All of the hotels do have portacribs available. Just let them know you need one when you make your reservation. Also let them know you will need a high chair when you make your dining reservations. Consider calling the hotel and asking them the best way for you to ship stuff to yourself before you go so you don't have to bring it on the plane and it is waiting for you at the hotel when you arrive.) Bring the baby's regular canopied stroller. If it is a dark fabric, consider bringing a white cloth to cover the seat before you put him in as this will help keep him cooler.
He will let you know what he needs, don't worry, you will have a fabulous time!