I have fostered two pit bull puppies, and neither have showed any signs of aggression. Even momma, who protected her pups when she came to visit was scared of our 30 lbs. and 20 lbs. dogs, and downright TERRIFIED of our neighbor's 10 lb. mutt. I would say you have nothing to worry about. Just get him socialization and training, and it won't matter if he's pit or not.
About your other concern, I don't think somebody is going to beat your dog. Do you live in a bad area? If so, possibly get a dog run with a roof, and a nice solid lock for the door. I understand you may be worried about dog-snatchers or something like that. Just watch your dog, maybe install security cameras if you really want to go there. But honestly, I don't think you have anything to worry about. Just get him micro-chipped and a few doggie friends, and he should be fine!
(Key part I forgot: Socialize him with your mom's dog leashed, until both you and her are comfortable with them on their own.)