Stop the high protien diet – most dogs are getting far too much as is and senior dogs do not need a high protein diet – it can lead to other issues and make it harder to get the extra weight off. He should be on a maintenance diet or senior dog diet. Getting the weight down will help ease the arthritis issues and other physical issues. Next, a thorough vet check even if one was supposedly done where you adopted him. Have a full blood screening run to check for organ issues (enzyme levels, etc in the blood can tell a lot about organ functions) as well as heartworm. A fecal exam to test for other internal parasites and a basic senior dog work up. Once he is medically cleared, all you can do is get the weight off, monitor and address any possible health concerns immediately as opposed to seeing if they go away, feed a high quality senior diet and appropriate exercise for his age and physical abilities (the vet can help you with this). And I also often do two physicals a year with my seniors as opposed to one and at one I always have bloods checked. Thanks for rescuing a senior 🙂