.... I can imagine the first, greatest factor in what I did next would be based on the response from the dog owner. Are they bending over backwards to show they are upset, apologetic, and demonstrating a clear effort to rectify this situation in a meaningful way? ....
To the OP - very sorry to hear of your son's injury.
Regardless of breed, the dog was A. put in a bad spot by its owner B. should not have done what it did. Whether through lack of socialization, or innate temperamental flaws, there is no excuse for this dog to grab a kid passing by & minding his own business. Is this a fatal error? I think heartmama's quote gives some guidelines. For both the benefit of humans & for the reputation of the breed, it does no one any good if this dog is put in a position where it might bite again.
I personally think Am Staffs & the like can be wonderful dogs - but that they are so popular, there are many poorly bred ones out there, so I view any really popular breed with a certain amount of wariness.
That said, we do have an ancient APBT type dog, of unknown origins, that I picked up on the side of the highway when he was 1/2 grown... Pip with one of our DDs
He's absolutely bombproof, but we always watch to make sure he's not being pestered. I also point out to my toddlers that all dogs can bite, & that they must be careful not to scare, hurt, or annoy them.
I do really worry about dog interactions (with strange dogs), since my DDs are used to our dogs, & I do my best to teach them about animals, but they're little & it seems like a toddler's instincts are to do all the "wrong" things (get in a dog's face, hug, get in their food, scream & run, etc.)
Anyhow, just my somewhat random thoughts pertaining to this discussion.