can l apply for multi-entrance visa to china for my son he is 13 years old ?

You can apply for multiple entry visa to China for your son at the age of 13 years, you don't need to take him with you for aplication when you are in Hong Kong. However, can you tell me more information about your son, you and your spouse?

Generally speaking, the multiple-entry tourist visa to the following applicants:
1. Foreign spouse and children of a Chinese citizen
2. your son was born in China
3. your son has visited Mainland China twice in the last year