How old is he? Obviously if he is fairly young to state the obvious he may not have grasped the concept of counting yet.
If he is an teenager/adult then yes I would be worried - the test would be to give him 2 pound coins and tell him its 21pounds and see if he complains !
does this boy climb up cupboards for nutela,and sit as close to the tv as possible watching lazy town!!! does this boy close his eyes when he see brunettes!!!