If I could keep my existing passport, the HKSAR one would be quite a useful one to have.
Lots of HKers hold passports from other countries but HK government doesn't allow those who want to become their citizens keep the passports from home country, doesn't seem fair.
You don't need to consult an immigration lawyer really. Just go to the Immigration Department (or call their English hotline) when he has the Permanent Residency HKID card and ask them how to follow up with renouncing the US passport and applying for a HK one.
I never know you have to give up your own passport to get a HK one. It is quite unfair as I know a lot of HK citizens have 2~3 passports.