puppy aggression

Question
my nine month old boxer is showing aggression towards people and other
dogs while on walks. he typically starts to growl or nip when i try to call him
back or or after the introduction is over. I cant get him to ignore people by
asking him to "leave it" or "watch me" he just focuses harder.
I've tried my best to socialize him, he's well exercised (1 to three hours of
walks per day), neutered,and spends half the day in dog daycare running
around. He hasn't bitten anyone yet but I'm beginning to lose trust in him.

Answer
I would say this isn't so much aggression as brattiness; nine months is the beginning of adolescence and a typical age for Boxers to test their limits and think they're "all that".  This is the time that you need to be fair but firm, and remind him that you set the rules and you control the resources.  If other people hold his attention on walks more than you do, then you need to become more interesting - find better food, a super-favorite toy, a silly game, anything that will get his attention on you so that you can reward him heavily for it.  You might also institute the "Nothing in Life is Free" protocol for a while, until he gets past the bratty stage and starts behaving like a well-adjusted young dog again.  

An article on NILIF:
http://k9deb.com/nilif.htm

And links to several on different ways to teach attention/focus/watch me:
http://www.k9events.com/clickerD_E.htm#attention

If things get worse, you can also work on desensitization and counter-conditioning - several articles on that here:
http://www.k9events.com/behaviourD.htm

The worst thing you can do is "correct" him for this behavior - if you need to walk backwards to remove him from the situation, that's one thing (there should be consequences to misbehavior), but if you use physical or verbal punishment he will very likely learn to associate the other people/dogs with the 'bad things', which will make his aggressive behavior worse.  Focus on what you want him to do, and teach and reward for that.

Good luck!