Even big dogs can do extremely well at flyball. We have a big Lab on our team that is suprisingly fast. German Shepherds, Labs, Standard Poodles, German Shorthaired Pointers, Golden Retrievers, Entlebucher Mountain Dogs, and Giant Schnauzers all run successfully in my region. NAFA has a whole division of racing called Multibreed where every dog listed on the lineup has to be of a different breed. If your Boxer ended up being reliable with even a halfway decent time, there would likely be a spot for him in a Multibreed lineup.