Here are my thoughts. I have a 9 year old girl who is skiing the course. I too am a bad coach and not that great a skier and I have looked to this forum for help. Here are my tips in no particular order:
FWIW, my daughter is running 25mph long line now, but 10 weeks ago she was struggling at 17mph, so they can learn really fast.
1. Use a full line (15 off is harder for them) and the confidence they gain by getting all 6 regardless of the speed/length is huge.
2. Buy a kids weight rope...it doesn't drag on the wake and seems to help with slack.
3. Get him to work on making one smooth hard pull all the way through the wakes...they can actually cut right through the huge wakes without getting thrown off by them...they just need the confidence and practice to do it. This seems like the single biggest thing to me. the turns come on their own. I have my daughter stand in the back of the boat with the handle attached the pylon and practice leaning against it in a stacked position. Then I send her off to ski and tell her to feel the same way on the water. It seems to really help if she did it right before she steps into the lake.
4. Get him skiing with another kid who can run the course a little better than him. We ski with kids of similar ages and as soon as one kid pulls off a pb the others are always quick to follow (within a couple sets generally)...it helps them realize what is possible.
5. Keep it fun at all cost. It is great to see them excel, which they do so quickly, but it is easy for them to start to think a good day is a good buoy count rather than just having fun skiing.
6. Free skiing occasionally can really help to work on technique.
7. Buy good equipment. I had my daughter on my old ski until this year when I bought her a new one. The new ones are just better than the old ones.
Good luck. Hope it helps.