I’m hoping you know that with a Green Card you have to file (and pay) US taxes on your worldwide income. Luckily for you Hong Kong signed a dual-tax agreement with the US not too long ago, so you will get credit on your US taxes for whatever you have paid to the Hong Kong Inland Revenue. That works as long as where you are coming from has higher taxes than the US, but it really bites when you come from a lower tax place!
For FAFSA, you have to put down all your assets, but the value of your primary residence is not included in the calculation. So if you buy a place in CA, the value of that house won’t count in your assets- but if you still have a place in HK it will count as an asset.
And as a reminder, even if you and your son move to CA and buy a house it is not automatic that after one year you qualify as in-state. CA reviews each case:
It doesn’t mean you won’t, either. Just a caution.