There is no such thing as an agressive puppy when it's that young. He's a boxer, isn't he? Boxers aren't barky, but they are big on growling and making other noises. As you said in your other post, he's never played with other puppies before. Puppy socialization is VERY important and plays a very crucial role in teaching a dog that biting really hurts.

I've found most training schools to be around $100 a month.