回复：请教，请有类似经验的朋友帮忙

I think I have knowlegde to answer your question correctly :).
The specialist my boy has been visiting is super.
I saw a lot of boy/girl before/after pictures from an album in his office. His work is marvellous.
You need to wait until the Men Ya change. By that time, if your son is still underbite, then you have to wear the mask (that is the best choice).
Anything is about the jaw size. If the jaw size is small, you need to enlarge the jaw by wearing a retainer. All those thing should be done around 7-8.