12 1/2 is not old. I would not feed a senior 36% protein however (I would not feed my younger ones that high a protein either)
Any good senior food he will eat should be fine Some that come to mind that are not senior but fit into the parameters are Solid Gold Mmellenia, Solid Gold Hundeflocken, Annamet reduced fat, Prairie beef or chicken, BilJac kibble and BilJac frozen.