An average weight for a puppy is a very difficult thing to measure, that's probably why

They grow at such vastly different rates, it really is difficult to give a meaningful indication of what's "normal" for a given age.
Generally speaking, boxers grow upward until they're around 14 months or so, but don't finish filling out until they're two or even three years of age. At 6 months and 50 lb, your boy sounds quite large, but he could also just be a fast grower (just like human kids, some grow quickly, some more slowly). His final adult height and weight will be determined by his genetics - and the best guide you have is the size of his parents, grandparents and any siblings.
Ideally, his final adult height (measured from the ground to the withers) will be between 23-25.5" and his weight between 66-70 lb.