Has this dog been to a vet lately?
My first question is is the dog painful? Like if you scratch him behind the ear does he suddenly try to hurt you? Pain causes even the nicest dog to bite at the hand that feeds. It may have been that your brother went to pat him on the head--normal enough--and the dog snapped because maybe he has an ear infection and we all know those hurt. So the first thing I would do is get him a good physical examination done and see if that doesn't help.
You've mentioned food aggression which is pretty common among many dogs, not just pit bulls. He probably wasn't in the shelter for TOO long but I'm going to guess he wasn't by himself in his kennel and probably had to fight for his food. Let him be alone when he's eating, plain and simple. It's never a good idea, no matter how great a temperament the dog has, to mess with them while they're eating. Make it the most peaceful time you can even if that means putting him away in a room while he eats. If he has a bone or even a toy, make sure nobody messes with it. If you're in public with him, he doesn't need anything like that. Keep him on a short leash and a good collar or harness.
Behavior training is a must. If you can't go somewhere for it, you need to at least get a good how-to book and educate yourself. Pit bulls are fantastically smart dogs and pick up new things quickly. If you don't have the time for that, then the best thing to do is rehome this dog. Whomever you rehome it to need to know anything you can tell them--biting, food aggression, and so on. There are plenty of people who do have the time and resources for a "project dog" like this.
And lastly, if you can afford a behavior consultant with a board-certified veterinary behavior specialist, I would go that route. It gets pricey depending on your area, but it can be well worth it if you want to keep this dog.