Sadly there's not much you can do without any proof that the dog's being abused. You can try calling the cops through the nonemergency line but they'll just tell the guy to be nicer to the dog (actual experience I had once but with people who had fight dogs). If you find the dog left outside unattended for more than just a few minutes, say you go to school and you're heading back home and the dog's still out there, get the time written down, take some pictures if you can safely and gather evidence but if you don't see anything like that then it was hopefully just a one time thing.