Ok I have thought about it. I kinda jumped on your ass hard. Truth is you probably just don't know any better. You probably haven't done any damage yet. Bottom line you need to take him off the juice.
It will do him no good right now. Your dog is still growing. He is not grown physicaly or mentaly. Give him untill 18 months at the very least. I suggest 2 years. That may seem like a long time, but thats only another 4 to 8 months. Then again, that may not seem like long at all to you, and you might not see the difference. The difference is dogs do not grow at the rate of human, in the next 4 to 8 months your dogs mind and body will grow substantially. There really is no need or justification for juicing him right now. It WILL do damage man.
When he is grown you can juice him. However walking your dog and juicing him won't have any effect. Even if your run him it will have no effect. He MIGHT get a little stronger, but he sure as hell won't get any bigger. He wont get bigger if he does no activity, and running/walking/playing around in the grass won't make him bigger either. Also good food is key. But I seriously doubt you are going to feed your dog human grade food, so really lets not even get into that. At the very least get him the highest protein/fat/moisture content you can if you do juice him in the "future".
Truthfully juicing your dog is silly unless he is old, or you are going to dedicate time, money, and energy to get him in good physical shape. Oh, I forgot to mention time, money, and energy for the REST OF HIS LIFE LOL. You juice your dog and he gets big, he'll get small just like a human will. Your dog cannot compete in any sport that juice would be appropriate for. Simply because he is a boxer. They do not hunt, fight, pull wieghts, run, or do protection work. Other dogs will blow him out the water no matter how much you juice him. Therefore anything you do will be just for you and him only. That is fine too, just remember your going to have to let him mature before juicing. Then you will have to do more than just stick a needle in his leg to get any results.