Do treat him for trush. I personally use bleach but the store stuff works as well. There should not be any odor from his foot. While you have his foot up, look down the back of it [over top the heels] the frog should not be higher than the wall. If the frog is is highest part, that is what he is walking on and he will be sore. He may need shoes to elevate the foot off the ground. Alot of ottb have flat feet. Finally @ his age you may be dealing with arthritis. Does he go completely sound on Bute ? He may just need an over the counter joint supplement to help him out.