Another thing, unless he's got all of his baby teeth, he might be older than four months. A real APBT pup can be maybe fifteen pounds at six months..
One thing that's kind of blowing my mind is you expected "a little" animal aggression. These are dogs created to fight to the death in a pit if necessary. That being said, they are also extremely smart and will pick up on the idea you don't want them to fight. The trick is keeping that control in high tension situations. The best thing I've found is teaching them to ignore other dogs and letting him know they're not a threat. It's usually never 100% though, you always need to be careful around other animals.
ps. his ears don't look that short and his head looks pretty normal to me. Do you have any other pictures??