Ps Of course if there is any swelling or its tender have it checked out .
Knees are funny things , and when children get growing pains it is the knees and legs that hurt,maybe he has also found a way of getting some extra attention so next time play it down ,see if he forgets it quicker.I doubt if there's anything wrong ..good luck
When he hurts his knee he is just playing around usually running or jumping
May I ask what he is doing to hurt this knee, is he running and falling or trampolining ?