thinks he's all that

I sincerely apologize if this has been covered...I looked around but didn't see if.  If there is a thread about it I'd be very grateful for a link or the name and this one can die.

Bruce is four going on five, he's always been very non aggressive...to the extent that dogs who were aggressive towards him always garnered a "what is wrong with you" type of response from him.  In the past year or so he's been in some different situations, one being a friend's boxer who kept attacking him at the dog park (he has isssues with people paying attention to any dog but him) after a couple of times Bruce was in a position where he was leaning on my husband when he got attacked and decided that was enough.  Thankfully he immediately stopped when told so and didn't get into a fight, but it was the first I'd seen him actually go after another dog.  He also had an experience where a strange lab that wandered into our yard snapped at him when Bruce accidentally knocked me down.  Again obeyed perfectly

Since those situations it's seemed that Bruce has got himself an attitude.  He no longer lets the bug guy in to spray the house, and recently he got his hackles raised and not only barked bur growled at a dog at Petco.  Again he stopped as soon as I said something but it kinda bothered me.

Have any of you experienced this with older boxers?  Is it just a confidence thing or a dominace issue that I need to pay attention to and address?  He still plays with other dogs and is fine...I just want to make sure I don't ignore a possible warning sign and end up with more of a problem than if I make an effort to address it now.  

He's also discovering more people that he doesn't like....most of which I greatly agree with...but he's not shy about showing it...not aggressive but makes it clear he doesn't like them...so far it's all been guys

thanks so much for all who read and/or respond to this...Bruce and I greatly appreciate it.