As someone who spent many years working with people with disabilities this is not amusing. If he has an IQ in the 60's this type of thing makes perfect sense to him. It is awesome that he is holding a job. Your Aunt must have taught him to be independent and had him do chores at home, etc which is the key to success.
I think the term OP meant wasn't amusing, but rather 'endearing'.
We went on a trip the the USA with the kids. I got out all the US coin we had stashed in the drawers for over 3 years, counted it, rolled it as best I could, and split it into two baggies, one for each of them. I told them it was spending money (it was about $20 each, and I marked the bags with the amount).
They rolled their eyes.
When we get there, on the last day, one wants to buy a souvenir, the other some snacks at the airport. I said, OK, get out your money and pay for it!. Turns out one kid did not bring it, left it behind thinking it was not actually money or something, and the other was surprised to see it turned into a softdrink and chips for him.
I mean, he carried through an airport, through a trip, and back to the airport in his backpack, and it was heavy. He thought I was just being mean or something... They were 12 and 14 years old.