Hi Shirley,
I think 1st of all your son must get interest with chinese. Get him some story books with hanyu pinying since you said his hanyu pinying is not bad. Ensure the books have colourful pictures to attract his attention. Read together with him. Try to speak with him in Mandarin. Explain some of the words how it form. Eg. 日+月=明,冲why only 两点水instead of 三点水because you don't want to waste too much water, etc. Hope this will help you abit.
I think 1st of all your son must get interest with chinese. Get him some story books with hanyu pinying since you said his hanyu pinying is not bad. Ensure the books have colourful pictures to attract his attention. Read together with him. Try to speak with him in Mandarin. Explain some of the words how it form. Eg. 日+月=明,冲why only 两点水instead of 三点水because you don't want to waste too much water, etc. Hope this will help you abit.