
Personally 6/7 miles every other day is far to much pounding exersice for a Boxer so young.
Boxers are a slow physically maturing breed (along with mentally) and for me , I would worry , about the impact on their joints.
Boxers along with many other breeds can suffer from hip problems, (although , and I know this will be a bone of contention for some Boxer people) they are one of the few large/medium breeds in the UK that are not routinely hips scored before breeding
I know to many Boxers with HD , my own included, so on that note I would advise curbing that amount of exercise.
A good run off lead in the park for his age should be suffice.