We had two sessions with some success. We started just working the beach start (both of us holding the boom) and trying to keep all things aligned and then i would let go and have him try to hold it. He got to the point where he could beach start a few times.
One challenge is the length of his tibia and the ammount of water you need to be in to clear the fin. Growing will help this problem.
I did not try the top down/reverse water start drill. but one thing i tried that has promise was me on SUP sitting slightly up wind holding mast tip and top of sail. I was able to help him position the rig and board through sail control and then had him try and keep it all lined up. I think this will be good. I will also add the focus on steering through mast foot that you mentioned so he gets the feel for moving the board to the correct position. No water starts were made but one came close. Overall successful as kids just need to be in the water trying stuff and they learn.
I will try and post back with other outcomes in the future.