On the bright side UPS did hire him and he will have that on his resume! It will be helpful - you'll see.
Maybe give us more detail about his positive and negative attributes. We collectively deliver to every business in the US and can probably give you a lot of suggestions about jobs that we think/know he could do.
I know grocery stores in my area hire people, who are like your son, to stock shelves. Our county admin bldg has a young guy who delivers interoffice mail and stands at the front door and directs people to the correct office. My cousin is developmentally delayed - drives a car, but can't count money, and has worked as a dishwasher/busboy at the local country club for 20 years. If these jobs I mentioned are below your son's skill level I apologize.
My son actually had two jobs prior to this, but because they were volunteer positions, I didn't count them (I guess I should have.) He volunteers on an ongoing basis for a local charity in their warehouse, loading packages onto trucks for delivery to low-income people. He then goes with the drivers (also volunteers) to help deliver them. Basically the same work a UPS driver's helper does (plus the warehouse truck-loading work).
The reference letter he was given upon request from the person who runs the charity was perhaps the best reference letter I've ever seen. All the warehouse workers liked him; they all commented that he was very fast, very helpful, very friendly, and very strong at lifting the boxes (which weighed about 30 lbs each, several at a time being carried). Another volunteer position he does involves standing for 14 hours straight, very quickly assisting people. The reference letter he got for that position was also excellent. Since both positions involved work similar to what UPS workers do, I felt certain the job would be good for him. Maybe this is why I'm confused.
However, since starting this thread, I have spoken to other UPS driver's helpers, and many told me that they were hired to be "on call, as needed", and they think my son was too, esp. since they hired him late in the season. They said UPS does not tell you that upfront, for fear you'll walk away and then they won't have anyone in case a driver's helper drops out, quits, gets injured, etc. I am sure the lack of work was not due to my son and his abilities, because of his past volunteer experiences.