Wow did that(OP) bring back memories. I had a customer just like that once. I was almost to the point of tears. I worked a very late shift in a diner, alone with just the cook.
When my guy started in with the rude comments, I had about 3 other tables. They were telling him to shut up. In fact, one tiny teenage girl stood on the booth and told him to leave me alone! It got really tense. I was about to call the cops.
The manager was gone. It was just the skinny cook and me, so the cook finally came out and kicked them out. It was also a husband and wife. But in my case the wife looked very scared too. I see them around from time to time, and the guy still gives me a death stare. The scary thing is that he has two kids now, and I hope he isn’t mean to them! He just hated me from the minute I said hello, and I have had never had a problem like that before, or after.
The funny part, after the fact, was that the food he complained about, (everything) was half gone, and when he was kicked out he was trying to take it with him. We didn’t have them pay anything, and he was picking up all his food in his bare hands and trying to take it!!! The cook said, “If it was so horrible, leave it and go!!!”
I hope your son doesn’t take it personally, and can try and forget it.
Some people are just awful.
p.s. My rude guy wasn’t diabetic, just an ass.
I do think now, that he wanted a free meal, and also just got off on being abusive.