Hi, Caitlyn! Traveling to Walt Disney World with a young child can be overwhelming - both in terms of excitement and concern! We got into some habits when my girls were very young that helped put my mind at ease, and that enabled the whole family to have a much better time.
If your young prince does not know your cell phone number, make sure he has it on him and knows where it is. I used to use a Sharpie type marker to write my cell phone number on the inside of my daughter's t-shirts or on the brim of a floppy type hat. If we ever were to get separated, she knew she was to go right to a Cast Member and show them the phone number. They also now have clips you can put on shoe laces that will contain the appropriate information in the event Mommy gets lost.
I bring my own hand sanitizer and toilet seat covers. I'm not germophobic by any stretch, but it's just sometimes easier to have all of my own stuff with me. We use the sanitizer before meals and after we wash our hands in the bathroom.
Make sure your son is well sun screened - even the tops of his ears! The Florida sunshine can be hot, and it will find the most unlikely places to put a sunburn!
Drink plenty of water, and keep a few snacks in a bag with you to help pass the time in lines and to tide you over between meals. I also find that having some crackers or something encourages the kids to keep drinking, even when they think they aren't thirsty.
Keep close; stress the rule of hand holding; and have a wonderful vacation!