I wouldn't waste my breath on the owner; you gave him a chance and this is how he chooses to act on it.
I would call the police but, be prepared for them to not do very much, if anything.
If would be great, as has been mentioned, if you could find him a great home. Pit Bulls are the first in line for the gas chamber.
I was only going to foster a Catahoula Leopard Cur/Fox Terrier mix but he looked so much like a Pit Bull at, at first glance that nobody would take him. Had I given him up to AC, he would've been put to sleep.
He is without question, the most gentle-natured non-aggressive dog I have ever owned. My cats beat him up and he is a 90 lb dog.
I also agree to feed him and keep him gentled until other arrangements can be made. If you shoo him away, others will do the same and too much of that may possibly put his back against the wall to where his personality completely changes.
Thank you for helping him and best of luck in re-homing him
P.S. Naturally he isn't neutered -- "they" never are and it's most likely why he disappears several days at a time. I'm betting there will be a lot of pups in your area in a few months