My son hold both Japanese and American passport, how can he get travel visa to China or enter China without visa?

Hi, I have a question for you regarding travel visa for China.
My son is 17 years old and he is a duo citizen, Japan and America.
he goes to school in US (currently living in US ) and he is going to visit his friend in China for a week in next month.
He has both US and Japanese passport but since he is flying to China from US, not from Japan,
I was wondering if he needs to have a travel visa to China as an American?
or he is still able to go to china with his Japanese Passport without any visa?
Oh, one more thing, he went to China from Japan about 2 years ago and
at that time, he entered to China with his Japanese passport without any visa.
Since he has US and Japanese passport, it is very confusing.
I just want him to avoid any problems when he enters to China or when he re-enters to US as well.
please advise.

Thank you very much in advance.

Naomi Yamada