It could be developmental or he's got a slight cold or similar.
It could be a cold as DH has a very bad chest cold and DS has been coughing just a little bit plus a few more sneezes than usual. He is on a Homeopathic remedy called ABC that was provided by our amazing herbalist. Maybe along with fighting the effects of the AB's he is also fighting a cold.

Poor little man - he is being so strong & amazing!