If the dog did not come from papered parents or the papers where never sent in for this dog. you will NEVER know the bloodline. Period.
Underbites are a sign of bad or tight breeding its not a bloodline trait. 44lbs is a good weight for a true APBT
most lines with produce dogs around this size.
Just enjoy him for the dog he is, because you are not going to be able to find out his bloodline unless you can find out if the parents where papered and what their papered names and registration numbers are/were. Sorry