If he continues the progression he is
in now, he will always be the youngest
in his class. Unless he is a genius, this will cause
academic problems later on. I would suggest you try a trial placement at the younger level and see how he responds. Is actions at home seem pretty natural for a boy of this age. In fact from now till he is about 7 or so, he will be a very interesting handful. Love him and enjoy him while he is at this age because it is really a very special one.