At this stage, keep it fun. Try teaching him without grilling him on Form. That kind of stuff can come naturally. Take him stump shooting and shooting at fun targets like balloons floating around the yard.
You may not want to hear it but, he may listen to your friends better. Take him out with a few guys. If he idolizes one of them, he will hang onto every word.
When I was a ski/snowboard instructor, I would accomplish more with kids in an hour than parents could in a whole season with their kids. Once the kids feel like their parents are getting frustrated and are just constantly telling them what to do, they build a wall and stop listening or try to do well and get frustrated themselves because they cannot do well. Its a mental thing and having people who can try a new angle and a new way of teaching is often the ticket.
But at 9 years old, I presume he is not hunting. The main thing for him is to have fun. They will be able to overcome bad habits far faster than an adult. If he is not hitting the target, he will lose interest more quickly.
Also , allow him to shoot from wherever he wants. Try to encourage super close shots. Pretend you are frog hunting and shoot at mushrooms and stuff. Join in with him at taking the close shots. Its fun watching mushrooms break apart with a big judo or rubber blunt.
Be careful on the close shots that you pick targets that are not so hard, that the arrow bounces back.
You may be surprised that eventually he will develop pretty good form on his own. It's not to say suggestions shouldn't come once in a while from you. Give him some basics but do not constantly drill them in his head.
Most of all enjoy your time with your son, I hear they grow up fast. The more enjoyable you can make this for him, the longer you will be able to share archery together.
Also put a big emphasis on making sure his arrows are properly tuned for his bow. better flight will make for better shooting.
To give you an idea where an eight year old is at, my nephew Oliver and I celebrated hitting the target with every arrow. It was a big accomplishment and we made it a positive thing instead of worrying about hitting the bullseye.
Far from Perfect but he walked away proud and excited rather than frustrated and burnt out.
dont expect anything from him other than safety.