Boxers are technically a medium sized breed, however large breed foods were formulated to encourage slower and more even growth. With fast growth we see more joint and developmental issues. Dog Food manufacturers consider any dog over 50 lbs to be large breed.
It is thought that high protein diets can contribute to joint and development issues with puppies. Puppy formulas tend to have more protein and fat in them which can promote faster growth in puppies leading to some of these joint problems and issues later on.
The large breed puppy foods are formulated to help slow down the growth.
I personally just have always avoided puppy formulas.
I would feed an "all stages" food from the start, somewhere between 22% - 27% protein.
Canidae and Chicken Soup are both excellent foods. If it were me I would just start the pup on whatever you are feeding Charlie now.... The Chicken Soup for the Pet Lovers Soul is an excellent brand. That way you don't have to worry about buying two different foods and you can feed the same thing and hope the new pup does well on the same food as Charlie.
Good Luck with the new little one, and be sure to post some pics!
Lisa