One way to get an idea if your dog is overweight is by checking their ribs. The ribs should not be visible as they stand, but still detectable when you touch them or when they move.
Boxers need plenty of exercise and stimulation as they are an intelligent breed. However, not until they are out of the puppy stage to reduce Hip Dysplasia. Two walks spread over a day and plenty of space when at home.They should not be exercised for at least an hour after eating, to help prevent bloat. Walking also helps the Boxer to socialise with other dogs and humans and it's good training time too.
They are not the ideal dog for people who spend most of their day away from the home. Because they are intelligent they will find things to occupy themselves, most being destructive in nature, if they are left for long periods on their own.

There is growing concern that because of the various additives and the prime source of protein in many commercial dog foods, dogs are not living as long as they should. Recent recalls in the USA have led to many boxer dog owners looking at alternative diets.FDA Pet Food Recall

One of the interesting alternative diets that have been found to benefit both dogs and cats is one based on raw meat. The thinking is based on what dogs would eat in the wild. First, other than stealing from humans having a BBQ their food is raw, not cooked or processed. Also in eating say a rabbit they would also digest various vegetation and even carrots. Now this is the important part dogs are not able to eat chopped up raw carrot because their digestive system is not designed for it. The rabbits is and the dog would eat the rabbit, including it's degetive tract which would include the predigested vegetables. I know it's not the best read, but that's what happens - think about it, what do wolves or hyenas eat? Raw meat, bones and every thing else they can leaving the remains of the carcass for others to pick clean.

Boxer dogs