Yes, he might lose his green card if he is not living in the US. In theory, permanent residents can take a number of steps to preserve their permanent residence status if they want to live outside of the US temporarily. Those steps include applying for a [re-entry permit](http://www.uscis.gov/sites/default/files/USCIS/Resources/B5en.pdf) before leaving the US, or for a [returning resident visa](Returning Resident Visas) after the permanent resident status is considered “abandoned”.
In practice, it is up to the immigration officer at the port of entry (i.e. the airport) to decide if your son is a prima facie permanent resident of the US. If the immigration officer does not believe that your son has his permanent home in the US, he may deny him entry, despite a valid green card or re-entry permit. (This happens a lot when green card holders try to “game the system” and enter the US several times a year for short periods of time to maintain their permanent resident status, while appearing to have their primary residence in another country.)
If the immigration officer denies him entry as a permanent resident, he can appeal to an immigration judge who has the final say. However, if your son has never lived in the US for any length of time, and starts college in Hong Kong after obtaining his green card, it will be hard to make a case that he considers the US his primary and permanent home. (I have heard of people making a successful case when they wanted to finish a foreign degree they had already started before obtaining their green card.)