Hi Vanessa!
Thank you for bringing your question to the Disney Parks Moms Panel!
The normal Disney Resort bus transportation generally begins running about an hour before the first park is scheduled to open. That would definitely be an option for your son on the morning of the marathon, and possibly the half marathon depending on how fast you run and how early you will finish. It's pretty easy to get from the
Epcot bus stations to the finish area.
While the runDisney buses are mostly not ADA compliant, the Mears bus representatives at the runDisney bus locations will be able to arrange for a van to take him that is ADA compliant. So your son (and anyone in charge of him) could still use the runDisney transportation by asking for assistance. They should be sure to allow extra travel time for arrangements to be made.
I also always find it to be a good idea to stop at the front desk the night before and double check where the bus stop will be and let them know your situation. They may be able to give them a heads up and/or have alternate suggestions for you.
I hope this helps! Please let me know if you have any other questions.