I suspect he's acting out about the bullying by the younger dog. I think Murphy is right on the money. My old girl is 14 now and I am always trying to keep on top of the 5 yr old 'youngster' who would love to run the show. Mad at you? Maybe. Maybe an extreme way of saying 'these are my people, not yours' to the kid in the family. Glad you're working it out and not just writing him off as many would likely do. Wouldn't trade mine for anything; they have enriched my life immeasurably.