How to apply for Chinese visa for my son (15 and 11 year old)

my son is 15 and born in US. I am a US citizen when he was born. My husband is from Hong Kong and holds Hong Kong passport. He didn't get his greencard until after my son were born. I tried to get Chinese entry permit for my son before but got rejected cause they say since my husband was not a greencard holder when my son was born, my son needs to get travel document instead. However, my son do have his US passport, and has never been to PRC before. Can we issue a letter certifying he does not have any Chinese status and therefore, he can apply for Chinese entry permit rather than travel document?

Thanks,