1. USN's must enter the US as USN's even if they have multiple citizenship.

2. While a US Passport will speed OP's son through formalities, a USN cannot be denied entry for failure to present his passport. He has a right to enter the US.

3. OP's son will create world of long-term hurt for himself by trying to enter on another passport. All he need to is present himself as a USN, say that he forgot his passport and submit to the secondary questioning which will presumably confirm that what he says is the truth and he will be on his way.