Yes, certain features may be restricted in a country, but that depends on the region set for the Apple ID, not physical location. The Apple ID region is what controls this access.
If your son's Apple ID is set to the US, then it should be able to access all the features. Just as you would have access to your subscription features if you travelled there for a holiday.
If its set to Vietnam, then you can't share anything anyway as Family Sharing requires all Apple IDs be set to the same region.
If he subscribes to Apple One himself, then he would only have access to features available in Vietnam because his Apple ID is set to Vietnam.