Boxers are a medium sized breed, rather than large. You're right that some food manufacturers suggest "large" breed formulas for any animal that will grow to over 50lb. Some others don't suggest that until 75lb. And some don't make any distinction at all (on this last point - it is worth bearing in mind that any benefit to, or need for, special formulations for large animals is entirely unproven, its just a theory... and not everyone has jumped on that marketing bandwagon just yet

).
In any case - even if you do buy into the small/medium/large/giant requiring different foods concept... would you really lump a medium sized breed like the boxer in with giants like danes or mastiffs, against which it is a midget? LOL
And just to make things more complex - there is no standard difference between foods labelled for large breeds and standard formulas. With some brands, there is NO difference, except in the packaging (i.e. it is purely a marketing gimmick) - and possibly in the size of the kibble bits. Some other formulas reduce the protein or fat content (and yet some other
increase it).
Thus, I think the answer is simply not to get caught up in the whole large/small/medium thing. Instead, take a look at what is
inside the bag - it's a whole lot more relevant than the marketing material on the outside

Many large breed formulas are perfectly well suited to a boxer puppy, whilst some others I'd avoid. And the same goes for standard (or even small breed) formulas. So choose on content, and just ignore the packaging.