My Pit Bull Is 11 Years Old And Has Been

My pit bull is 11 years old and has been in good health; over the last several months he has been randomly vomiting, loosing weight but still has good appetite. Hi abdomen looks distended and now his spine looks different. At first notice I thought maybe worms so I deformed him, no changes. His mood is no different but energy level is less. He is an inside dog and never around other animals and loved member of the family. I'm afraid he may have cancer or something causing malabsorption. Any suggestions?