That is wonderful and almost unheard of for a rat to live that long nowa days! Can I make a suggestion here? Do check his teeth too because maybe he can no longer chew well, Are they too long or looking okay? Feed him some whole wheat bread, soya milk and some baby mixed cereal everyday and maybe some yogart for his tummy, Frozen green peas because all rats love them, a little honey is good for extra ebergy too. Children's Echinecia as well. Hope this helps a little and good luck with him.
Carole and her Lil Ratscals