My youngest delivered pizzas for about 4 year, through college.
One thing he said he could count on, a pizza delivery to a Black residence was certain to recieve no tip.
One of my kids’ friends delivered pizza and said that if people did not tip him for on time delivery he would tell them:
“If I was going to have my dinner sitting alone in the car with a guy for 15 minutes, I’d want that guy to know I cared about him enough to give him a dollar or two.”
A subtle warning, to be sure.
I did the Pizza delivery thing for a few years, and there were 2 groups that I could almost guarantee no or nominal tip.. 1 your son found, the other were the expensive upper middle class homes.
Blue Collar neighborhoods were generally the best tippers. More expensive the homes got the less likely a tip in general, and the poorer they got the same thing.