We took our 5 y/o DS on his first trip to WDW this year. He is a seasoned traveller and has been to many other places, so staying in a hotel was not an unsual experience for him.
I would make sure to pack a night light. Those rooms are pretty dark at night and if you son is not used to travel, strange rooms or really dark rooms, he might not like it.
I would strongly urge you to rent a stroller. Little legs last longer this way.
My son would have been completely miserable without his sunglasses and hat. The sun is much brighter and stronger than what we were used to in Ohio that time of year.
Take along little snacks and juice boxes or something. My son refuses to drink soda, so having a juice box with us saved us a lot of hassle.
Okay, I just asked my son what he was really glad to have with him at WDW and here is his reply:
"I really liked my necklaces--the pin one and the bead one. Oh, and tell them not to take their teddy bears."
The pin necklace was his pin lanyard and the bead necklace was a cheap little stretchy thing from wal-mart. He forgot his very favorite teddy at WDW and he never found his way home--hence the comment about the teddy.
I am sure there is more, I just can't think of anything else right now!