The past few days, he has started napping, a lot! He seems to be sleeping at night also.

I would say growth spurt, but he's not so interested in eating. He has no sign of a fever or cold, and doesn't seem irritable once he gets his nap.

He's about 5.5 months and more active when he is awake, playing in his exersaucer, sitting up in my lap or propped up with pillows, and rolling around a lot. Could it be that he's just wearing himself out? Should I be concerned?