IMO I would say that I don't think he's pure boxer, the shape of his snout and face just isn't the boxer shape, and like you say I would also suspect boxer/pit mix, but he definitely has the boxer colors.
id say a pet is a pet so papers dont matter and whether he is pure or not doesnt matter he is your companion so you should just enjoy that. It is always good to know lines of the dog but he is still a beautiful pup.
If I hadn't known better, I'd say def. mix. However, I brought my dog to the dog park, and this lady had 2 boxers.....one was 9 months old, and the other 2 years. The 2 year old had a long snout like your dog does, and didnt really look full boxer, although the colors and everything were that of a boxer. She saw both of the parents, and they were both full breed boxers. Your dog resembles this one quite a bit, although his nose wasn't quite as pointy, and he was brindle.