As I recall, it is his passport, the "invitation" letter and some other form of ID. Have him be the person presenting his documents and doing the conversing with the Officer. You should not interrupt unless something turns really ugly or there is a fact which only you can supply.

Bear in mind that for a 16 YOA, CBP is working off your records unless there is something peculiar about your son's background, e.g., he lived abroad apart from you for a substantial period of time.

Also, to the extent that there are any other documents which may be called for, you ought to have them (but don't present them or show them unless asked). Not likely for a 16 YOA, but those are things such as court records of dismissals and the like