Hi Kathleen! Welcome to the Disney Parks Moms Panel!
September in my experience is typically a very warm month at the Walt Disney World Resort. Temperatures usually tend to be between the mid 80’s to the low 90’s, typically with high humidity. However, there are plenty of air conditioned areas and some ways that you can beat the heat!
My first recommendation to you would be to try to tackle the parks early in the day, head back to your room for a nap or pool break mid-afternoon, and then go back to the parks in the evening as the sun begins to go down. If your son is in a stroller, consider getting a clip on fan or two that you can run while you are in the parks. I also bring with me a stroller sun shade (Summer Infant Rayshade Stroller Cover) that can also be purchased on amazon. This provides extra coverage if in a stroller from the sun.
All four of the Disney Theme Parks offer
Baby Care Centers. These centers offer a respite from the heat. You can feed and change your children here and the changing tables are full sized. There is a kitchen with a microwave and sink. There is a room with a television, sofa, and chairs for a little down time. You can even purchase things like juice and sunscreen.. These areas have been a lifesaver for us, especially with my son who has a sensory processing disorder. When he would get overheated or have a sensory overload, we would bring him here to just relax and unwind for a bit.
Lastly I would try to plan your days so that you are spending your daylight hours in a lot of the indoor attractions and entertainment settings. I would plan to spend a lot of time at places like the Sea’s with Nemo & Friends in Epcot, or taking in some of the shows like Disney Junior - Live on Stage! at Disney’s Hollywood Studios.
I hope that you have a fantastic time!
~Heather~