How did my son raise the cost of living there?
He was peddling stuff on the streets without a license and trying to sell his art, most households in Brooklyn had a lot more money than him.
The view is that your son, moving into Brooklyn, is willing to pay more rent than the previous group living their. However, he will also demand different types of services.
For instance, I suspect that your son would not be OK with open drug dealing on the front of his building. Having an increased police presence, with the added bonus of a jury pool who will convict, changes what is acceptable to the area and ultimately the culture.
What is happening is that the slums are actually being made to follow the rules and improve. I have seen it in small towns out west. It displaces some residents, but makes the area better.