Hi Darline! I just answered a similar question from another guest about wagons. Disney has had to restrict certain personal items from the parks to keep the safety of all guests under consideration. You can see these
Restricted items on the website and wagons are listed first. This also includes the foldable or collapsible wagons that are now being made.
Understanding that your son has health issues that prevent him from walking great distances it would be best if you considered renting a
Wheelchair for your park days. Each of the four parks will encompass a great deal of area so walking a lot can be expected. The wheelchairs would allow your son to rest between attractions and they are certainly roomy enough for him to be comfortable. The rental areas are at the entrance to each park just inside the queue areas.
If you feel that you need a wheelchair during your entire stay including use at the resort then there are many local wheelchair companies that could drop one-off and pick it up at your resort. Sometimes this is easier then having to stop and rent one each day at the parks.
Hope your have a wonderful vacation. I am sure your son is excited about this next vacation!