Hi Tricia,
President's Day is Feb 15th this year - that's typically a very crowded week. You'll be arriving at the end of it so hopefully the crowds will move out for you after the weekend. Feb 25th is the Grand Opening of ESPN Wide World of Sports so the crowds may pick up again. If you're planning to visit a park on the 26th, avoid DHS, as ESPN The Weekend begins there on that day. I'd suggest looking on the WDW website for the hours of operation & parks hosting Extra Magic Hours for mornings & avoid them. They tend to be more crowded & having a little extra room for your son may be helpful.
I know first hand that Disney is an awesome place for special need kids. They have a section on their website devoted to traveling with guests with disabilities - click
here. I've recently offered some tips & suggestions for people traveling with special needs guests. To see some of them, click
here,
here &
here.
Make sure you take a copy of your son's medical history - just in case. The evenings can get cool so be sure you bring warm clothes for your son. Go at his pace & don't wear him out. Take a day or 2 off from the parks to take a break from the crowds.
If you have any other questions, please feel free to let me know. I am very happy to help you make this a magical & memorable trip for you & your family.