I would definitely get the blood work done for tick borne illnesses first. If all that plays out negative you could consider xrays, but I don't think that's going to show you much if he's active outside.
Could it not just be that he's content to be lazy?? I love a nap on Saturday afternoons.
Our dogs have always been pretty content to laze around in the house if that's what our schedule dictates for the day. Now one is gone and the other is old and sleeps all day if you let her.