the first thing I will say is NEVER tell him to keep his head still or head down, once you do that you will mess him up for good. and if somesone ever says that too him you tell him it is wrong and ignore it. the head should flow as the body moves it

I have a nephew who is five and what I do is this,
go to the range and ask him to just watch me hit three balls, tell him to watch my preshot routine, set up, etc... Kids learn A LOT by just watching because their brain takes a metal picture and you tell a kid to "do this" and you show him then he will do it. a kid will not ask if the hips fire first or the arms dropp, etc......

from there I tell him to stand behind the ball, look at his target, walk up to the ball and look at his target and makes sure the club face points to his target. after that I leave him alone and let him have fun. If you want to work with him a little more between every ten balls he hits you have him watch you hit three, and you better do it right because you are the example

have him watch you, teach him a VERY SIMPLE preshot routine, and let him have fun. You might be suprised what he figures out on his own, or digs out of the dirt