Howdy Marcia!
Disney's Saratoga Springs Resort & Spa is great! I know you will love your time there!
The Springs might be a great option. It's close to the main lobby, great dining and High Rock Spring Pool with a zero-depth entry.
I would also like to encourage you to do the following: Contact Disney Reservations and ask them to note your son's mobility issues on your reservation. Ask for a Villa (and location) that will be the most accommodating for him. When you are 10 days out from your check-in date, contact Saratoga Springs at (407) 827-1100 and confirm that your request is noted. When you check-in, explain your situation again and confirm that you were assigned a Villa that will work well for you and your party.
You might also want to request a Guest Assistance Card at Guest Relations when you enter any of the Theme Parks. You will explain your son's situation to a Guest Relations Cast Member who will then be able to stamp the Guest Assistance Card appropriately. The Guest Assistance Card is designed to alert Cast Members about Guests who may need additional assistance. The intent of these cards is to keep Guests from having to explain their service needs each time they visit an attraction. Keep in mind that it will not allow Guests to automatically bypass lines. However Guests may be offered an alternative waiting area based on the attraction.
I hope this will help you in your planning. Your 1st trip to Walt Disney World!! I'm so excited for you and I know your son is going to have an amazing time!! (You are going to love it too - but there is nothing better than seeing Walt Disney World through the eyes of your child!!!)
Thanks for your question and please let me know if you need any further information. Have a magical day!!