I think it should also be noted that anyone who has a "breeding pair" or (in this case has 5) boxers that they continuously breed together would not be considered a reputable breeder. Not by Boxerworld's standards, anyway.
When looking for a pup, I would consider a reputable breeder to be a breeder that shows her Boxer, finds out what her Boxer's faults are, and SEEKS out a male that has attributes that offset her females faults. This breeder, on top of that, fully health tests her Boxer (OFA Hips, heart, thyroid, etc...) and wouldn't consider a stud that wasn't fully health tested himself.
Back to the original question though - I suppose there is no way of knowing whether your dog is 100% Boxer without doing a DNA test. He could just be poorly bred. But he's still cute as a button and I'm sure you enjoy him and couldn't imagine your life without him!
Good luck to you