The first thing to find out is whether it is pain or some irritation that he is having. Try looking inside the ear canal with a torch...and compare with the opposite ear.see if there is anything unusual .at this age foreign bodies are common due to kids tendency of putting stuff in their nose,ears,etc. Also it's important to know if there had been any cold or infections in the past few days

Does he have cold, ear discharge? In this age pain due to cold common. ear foreign body should be ruled out.

May be he is having cold which has gone to ear, resulting in Acute Otitis Media.