I just started learning table tennis last summer with my son too!
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As his old man, I think you can do at least the following for him:
1) Read more about table tennis, as most kids don't like reading, so you read more about it and you can tell him more.
2) Watch more, well, you may not have much time yourself as you are only 4x, but watching more and listening more from experienced commentator and players would help you appropriate the game.
3) Tape his practice and analyze the mistake, it is much easier to spot other mistake than doing it flawlessly yourself. So you can remind him about his mistake. I find that if you focus in one problem at a time, break the problem up and constantly remind him about only one thing at a time helps too.
4) Plan his practice for him, as kid usual don't plan well. Just playing ping pong 4 hours a day may not be the most effective way. He may need other exercises such stretching and flexibility, building muscle.
In short, you should become his "semi-coach"
And of cause, finding a real coach with good experience would be the best. (if money and time permit)
You may even go back China to find very experience trainers and take lesson with them and ask for practice planing tips la!
Just my two cents worth and for your information, my son and I are not playing well, but just want to share my view! And hope other C brother would comment more!