Extremely hard to convince me. Virtually impossible, in fact.
The problem is that teenagers have trouble conceptualizing what $168K really means (and they know that there aren’t real consequences if they don’t pay off a loan to mom & dad). Maybe you can show him what $168K, compounded at a conservative 5% rate, will be in 20-30 years. Is there something that he really wants? Trips abroad, down payment on a house, even most grad degrees, etc., would be cheaper.
Also, did he do well enough on AP/IB tests to get enough credits from Duke to graduate a year early?