I was hoping to get some responses back from you before responding but since I haven't, I did want to give you some information on the situation.
The first thing I would have checked is your dogs thyroid. Boxers often have thyroid issues and hypothyroidism can cause sudden aggression in dogs. REad about the issue in boxers here:
That should be your first step. Your dog is going to need intensive obedience training at a minimum. He needs to come when called regardless of the distraction. He should stay when put into a sit or down position as well. I am assuming he has had some training but will need more if he can not be recalled when he gets loose. The following site is helpful in helping owners train their dog. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
The next thing that needs to be done if you do not want your dog seized is to prevent his escaping from an enclosure. There are many ways of accomplishing this but most invisible fences are not enough for a dog that is motivated to escape. Many invisible fences are underground and those no longer deliver a shock or reprimand once the dog is so many feet from the wire, thus allowing a dog to "run through". They are pretty effective when coupled with a traditional fence as they prevent a dog from climbing over or digging under a traditional fence.
Now there is the instant invisible fence that gives a reprimand the whole time the dog is outside the transmitter range but a motivated dog will still run out and then back in. I would suggest either a traditional fence coupled with the invisible fence to prevent his escaping.
You should also teach your dog not to run out the door. This is accomplished with the sit and stay command. You can put a tether on the floor near the door and keep a leash on him. You will attach him to the tether have him sit and stay then open the door. If he moves he gets no treat. If he stays in his position, then he gets a treat of a hot dog sliver. You want him seeing that staying inside gets him more satisfaction with the treats than running out.
It takes a lot of work. An inside pet fence (invisible) can help keep him inside as well. Another option is keep a basket muzzle on him from morning to night on the off chance he will escape. It is a bit extreme but if that is what it takes to prevent him injuring another animal or person which would likely cause his euthanasia by animal control.
I have to run out, but if you have more questions or need more clarification, just respond and I'll get back to you.
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