2.5 yr old aggression or dominance?

Question
I have 2 male boxers. W. is 4.5 yrs old we bought him. Z is 2.5 yrs old and we got him from a lady who had other dogs and could not keep him, she told us that she suspected her teen age daughter of abusing ie. kicking him.  So, we couldnt let him go to the pound, and took him when he was 5 months old. Now, Z is seeming to show aggression towards people.  We were walking him and my husband let him walk ahead of us on the leash, a teenage boy came up and started to pet him and z "bit" him.  Not an actual bite, but put his mouth around his arm, but didnt apply alot of pressure, just enough to feel it.  Then tonight a man came to our door, and z pushed past my husband and went out the door and my husband said he tried to bite the mans hand, but he pulled it back quickly.  Z has never shown any other signs of aggression.  I am now realizing that he has been showing dominance, ie. pushing up against us, using his head to bump our arm until we pet him, and we usually allowed the dogs on our furniture, now I have been reading that that should not be allowed, as the dogs should know that those spots are reserved for humans.  We cannot afford obedience training for him, but I have read about the dominance issues, and have made a plan to implement those things to stop.  I guess my main question is, since neither incident did z actually bite to cause pain or injury, it was like he was warning them, what are suggestions such as training.  They do have a fenced in back yard, and a heated and air conditioned garage that has been "dog proofed" and made into their room.  Any suggestions, as my family love these dogs and want to train them properly, as we have now realized the error of our ways. But want to make sure, can you train a 4.5 yr old and 2.5 yr old male boxers?  Both have been neutered.  Thanks for any suggetions.

Answer
I'm sorry for the delay in this answer - my internet was down!

You certainly can train Boxers of any age. :)  The fact that Z has not drawn blood, or I would guess even caused a bruise, is actually very good - it means that he has developed bite inhibition.  I'm not sure this would be categorized at this point as aggression - from the description you give, it seems as if Z feels he was reacting entirely appropriately.  With the teenager, he may have objected to the 'invasion of his personal space', and with the person at your door, he might have been protecting his home.  Obviously these are not behaviors that we humans want our dogs to exhibit, but dogs don't always know that!  This could also escalate, which of course you do not want to happen, so it is something to start working on now.

It probably wouldn't hurt to have a veterinary workup to be sure there is no underlying physical cause - and if the behavior becomes more extreme or more frequent I would definitely do so.  

Training and relationship building could help quite a bit - if Z learns that you are in control, you are the leader, you are the one that will protect him and the household, then he may no longer feel the need to fill that role himself.  ("You" in this case means all the adult humans in the family.)  The Nothing In Life Is Free program is a way to be fair, firm, and consistent in enforcing guidelines for appropriate behavior, and in establishing your role as the person who sets those guidelines.  
http://www.goof.com/~pmurphy/NILIF.html

Since Z may in part have been reacting to what he perceived as rudeness, I suggest you read this article by Suzanne Clothier (well, I suggest every dog owner read it regardless) - "He Just Wants to Say Hi!":
http://www.flyingdogpress.com/sayhi.html

There are also a couple of books by Brenda Aloff that you might find very helpful; "Aggression in Dogs" and "Canine Body Language".  These can help you determine where Z's behavior is rooted, and learn to recognize the more subtle signals that precede a 'bite'.  Brenda's site is here for a little bit of information about her and the books:
http://brendaaloff.com/

I'd also recommend Patricia McConnell and Karen London's book, "Feeling Outnumbered", which deals with establishing roles in multiple-dog households.   This book and Brenda Aloff's are also available at www.dogwise.com.

If the behavior increases dramatically, and there is no medical reason for it, you really should contact a behaviorist who can evaluate the situation and help you manage or modify Z's behavior.  There are behaviorist directories on these sites: http://www.dacvb.org/about_us/diplomates_copy_1/
http://www.iaabc.org/consultant_locator_dogs.htm
http://www.animalbehavior.org/Applied/CAAB_directory.html

Good luck!!