He's six years old... it will come with time. I know when I first started I used to lunge too far. I am not sure if you have somebody his size or if you can do it yourself, but one drill that helped me was the jab drill.

That drill consists of you or whoever else taking a step backwards as he comes forward and catching his jab(assuming your both orthodox) with your right glove. Do this in one direction, then have him catch your jab on the way back. You can do with the cross as well. Its really basic, but it teaches him how to catch the jab and how to move into it. After he gets used to it, just do some "sparring" where he throws one jab, then you or whomever else throws one jab, it will teach him range and timing.

Other things that help with learning the proper movement are combinations, if you have a heavybag, have him do basic drills where he moves into punching range, throws 1,2,1,2 then either dips and moves out or just moves out then comes back in. The reason combinations are good is that you have to have good movement if you are going to throw multiple punches and retain your footwork, and you have to have the footwork if you are going to be able to throw punches in bunches while moving. The key things to look for, and I am sure you know what to look for, but proper body rotation into his punches and making sure he stays balanced. If he over rotates he won't be a fluid puncher/fighter, and if he under rotates he won't get as much speed or power into his punches.

Like I said at the top just remember he is six, boxing is about mastering the basics, just make sure when you are working handpads and the heavybag you are making him move his feet. At that age its up to you as the pad holder or whatever you want to call it, to dictate the range on that handpads, don't be too far away, but far enough that he needs to take the right size of step to get in. Movement is like over rotation, its better to take two steps quickly than one big clumsy step. You know this already I am sure, but I just thought I would share it with you.