Aggresive 6 month old male bully

Question
my male bully pup has gotten a little more aggressive in the last few months but in a confusing way. I have a 21/2 years old nephew which my dog loved to play with when he was little, but now that my dog has gotten older he seems to have some aggression to children, he doesn't bite or even bark, but his fur stands on end, and he tried to bump the children away as if he is afraid of them.  he is un neutered and it is not an option because we will be breeding him when the time comes, i was just wondering what could be causing his aggression, and if behavioral therapy/training would be the right answer. he is fully trained in the basic commands (sit down stay shake go) and some other unnecessary ones( left paw, right paw, touch )

Answer
He's reached the same age as the nephew and now sees him as a competitor. He is a very dominant male puppy and unless you're willing to work this out with a trainer, I would have him neutered asap. This type of aggression toward humans isn't something you should choose to pass on to his offspring.