Ok I actually have some expertise in this area, not just in rugby league kicking but soccer, I coached both (kicking in league).
First of keeping your head tucked and eyes down will have one result and one result only, the ball will go lower, in fact if he did this more often than not he would struggle to get it over the crossbar.
You are correct about the left foot he plants it wide at times, there are two reasons for this that I can see, one I think it's from an injury, if you watch closely enough he walks with his feet in 10 - 2 position and a slight limp. Probably Achilles, or ball of his foot.
The other is he is trying for too much power, the follow through is important only in that you need to judge it correctly. A league ball is not like a golf ball and reacts differently to follow through, in fact follow through can tend to roll the ball as your foot pushes round it slightly.
If you are right footed, and you try to kick the ball too hard, the ball will always go to far to the left, this is because you try to push it too hard and plant your left foot too wide as you try for too much pivot and power. The effect is you lean too fat to the left and your foot goes too far round the ball to the right......
What he needs to do is go for less power. Try if your self kick a ball trying for the hardest you can and see what happens, then try kicking it not going for any real height or distance just for a place don't strain yourself just kick it.
It will be sooooooo much more accurate.
Unfortunately there isn't a cure at this age really and I'm happy enough with his kicking not the best but not the worst. If sooner have him in the team kicking some goals than have hodgkinson in the team at all.
That's not the best description but it's more or less what is happening.
He does need to lean forward a little more and more upright but that is more about him going for distance