It will be fun, really! WDW is *SO* much more magical when you can see it through your child's eyes.
The flight from the UK will be the hardest part; I've done it several times, but in reverse. Going home he will sleep, but on the way here he probably won't sleep much. I don't know if you have done time zones with him before, but IME, kids that age adjust at a rate of about 1 hr/day. He should be back to normal by your sixth day here. Until then, tread softly, and don't go anywhere without your buggy. If you can bring his usual carseat on the plane, do it; he will be much easier to handle if he is in his familiar seat. (You may have trouble with doing this, though, depending on what seat you have. UK seats don't work on aircraft unless you have one that can be secured with a lap belt only.)
If he has favorite biscuits, juices, etc., bring them with you if at all possible, or order them through a US internet supplier to be delivered to your lodging. Babies are funny about what they will and will not eat, and they like the familiar. (Speaking of kiddie food, be aware that you will receive very odd looks if you feed him a soft-boiled egg in a restaurant. Americans don't eat eggs soft-boiled very much nowadays, because of the Salmonella risk, and it is considered especially dangerous to give them to children.) The milk here is a little different than yours as well, it may give him some tummy upset just at first. Bring some of your usual over-the-counter medicines for him; you don't want to be trying to figure out the US equivalents whilst dealing with a screaming child. (Can't you just tell I've lived all this the other way 'round?
)
Bring the hood for your buggy. (I put mine in the suitcase until I get to Florida. Easier in airports that way.) I have seen so many UK families searching through shops in Florida looking for sunshades because they forgot the buggy hood.
Other than those few points, just follow the usual advice you'll find on this forum, especially the part about taking mid-day breaks! Best of luck!
