Hi Pauline,
What a great gift from the Make-A-Wish Foundation! It is one of my favorite charities. If you want to travel before the hot summer months, I'd recommend coming in late April, May or early June (just keep in mind that May 31st is a national holiday in the States & the crowds at Disney will be very high that week). Mid-September through October are nice months to visit as well. A lot of the pools are heated if the temperatures drop too low.
June through Sept usually brings a lot of afternoon showers around Disney World so if you chose either of those months, be sure you pack rain
gear or be prepared to purchase some once you're in Orlando.
Disney is an incredible place for special need kids and will do their best to accommodate your son's needs. They have a section on their website devoted to traveling with guests with disabilities - click
here. Make-A-Wish may be handling all of your arrangements and needs but if not, please feel free to submit additional questions with anything you want to know.
We're happy to help you make this an amazing trip for your son!