✔ 最佳答案
1) 100% sure he can stay in HK and at first he can get an temporary HKID card if you are Hong Kong permanent resident.
2). I m not sure if he will has to pay tax to USA in case he will stay and work in Hong Kong. This you should check in USA or he should know about how is the law in USA.
3) As i said if you are Hong Kong permanent resident he can go to Hong Kong immigration centre to apply one temporary HKID card with one star.
4) About working visa you can check
http://www.immd.gov.hk/chtml/topical_3_4.htm