First, bless you for rescuing the pooch. Bravo.
Second an active dog breed like boxers needs more than just attention. He needs:
You can do it three (common) ways.
a) If you are a jogger, you can work him into it...but you can't just start him out at 2 miles. Break him in slowly. Once he's in shape, he will be a great companion on your runs. This is the most efficient way, and the dogs love it. They will remind you every day that it's time for their jog. (If you are a world class athlete who is going at 5 minutes a mile, it may not work. Jogging works...but full out running is actually too fast for most breeds.)
b)Take him for walks every day. This means at least 30 minutes, and preferably 45. The faster you can walk, the better it will be. If you can do it morning and evening, even better. (In some big cities, you can hire dog walkers to do it for you. If you don't have that, you can often pay a junior high school kid to help for a moderate fee.)
c)If he has any retrieving instincts, you can throw a tennis ball (or frisbee) for him. There are even devices on the market that will throw the ball farther. WARNING: note that this is like doing wind sprints. The dog will get out of breath quickly, especially in hot weather. My experience is that it's best to limit these sessions to perhaps 10 minutes....but to do them a couple of times a day. Again, START SLOWLY! A link to one type of the tennis ball throwers is here.
A dog's mouth is his hands. Just like there are people who have to be doing something with their hands, there are dogs that have to be doing something with their mouths. We have had several like this. The BEST solution we've found is something called "NYLABONES". They don't hurt the dog (we've been using them for 2 decades), and the dog seems to think they are special. Our experience says do not get the "gummybone" or "flexibone" type, get the regular nylabone. Here is a link
where you can read about them....most dog supply houses (here is a link to one company
)carry them. We have dogs of a similar size to a boxer and we get the "giant" size. Both the chicken and ham flavors work well. The others haven't been very successful. They dogs actually chew up the material (and yes, they swallow it) but it is inert and doesn't hurt them. Some of our pups will go through a giant size bone in a month. (NOTE: our opinion--from personal experience, from out many dog colleagues, and from reading research articles---is that things like pigs ears and hoofs, as well as most rawhide toys today are NOT good for pup. The bulk of these materials today are made in the third world, and often contain noxious chemicals. Short term exposure might not hurt much, but long term can. Further, the hoofs/ears etc. have been found to collect in dogs intestines and cause trouble. I therefore would recommend you stay away from these type products.)
A second solution that helps (as long as they have the chewing bone also) is to give the pup something of yours that becomes his own. An old shirt, and old pair of shorts...something. (Don't give it to him clean....give it to him after you've worn it.) You give it to him and watch him
. Let him have it for a couple of minutes at first....or until he starts to chew it. If/when he does, take it away, give him the nylabone. Repeat every day, and you will find he can hold it longer. After a while, the clothing becomes like Linus's security blanket. We find that the pup uses it to suck on when they are stressed, but they don't chew it. We have to look around our house and pick up the items before people come over or we get comments like "why is that pair of red boxer shorts on the stairs?".
Your dog is now in the middle of adolescence. As such, it is his instinct to try and become the leader of the pack. He doesn't mean any harm by it, it's just instinctive. But it's a very bad thing. Human beings MUST be the leaders. We have found that dogs do not care where they are in the pack (after some instruction); they just want to know their position. We typically have 3-4 active retrievers at a time. The pack order is the two humans first, and then we let the pups sort it out among themselves. (We, the pack leaders, insist that they do so nonviolently, and it works.) The dog at the bottom still gets love and attention, and it never seems to be a problem. However, every (and I mean EVERY) male dog we've ever had, at some point between 14 months and 24 months, has decided to test us and see if he could just possibly become the pack leader. When it's made clear to him that he can't, he just says "no problem, fine, ok".
One way to establish the dominance of the people is to train your dog in obedience. If you aren't experienced, then take an obedience class...in fact take several at different levels as your dog learns. WARNING: There are a fair number of lousy dog trainers out there giving useless classes. Ask for references or ask your friends. Did the class help? Did the class teach both the owner and the dog? And make sure you keep the rules in place at home. (No slacking!) Dogs must sit quietly while their food bowl is set down and wait for a release command before eating. (Since food is really important to most canines, this simply excercise really helps.) Dogs have to sit and wait until you open the door. Dogs have to heel when walking, etc. Will they be perfect...nope. But by you demanding that they follow your standards, they learn pack order. (And, I might add, they
actually often learn to love doing obedience training. All ours have.)
You have a male dog of an active somewhat large breed going thru adolescence. What's happening now is expected. Gain control now, and both of you will enjoy your time together much more.
Good luck, and again, thanks for rescuing the pup.