You had gone extra miles in teaching your son Chinese in US and he was very much prepared for the new life in Beijing. Congrutulation!
My two boys go to Sunday Chinese school to learn Chinese for 2 hours every Sunday. In addition, we speak Chinese at home as much as possible. I hope they will be ok when moving to China even thought it is going to take some time.
I wonder other than language, if your son has experience other barriers such social, environmental adaption. If you prefer please email me [email protected]. Thanks!