MSPeconomist

Keep (print/save) as many documents as possible, including "evidence" regarding whether business class was full on the segment that was upgraded. If he can take photos of himself showing that he was on particular flights (with time and date) or photos of him at boarding gates with flight and time displays, that would be good too, but don't violate AA rules about photography. If you have evidence of fares available at various times, try to save that too, including information on fare class availability from EF (which is also useful to check fare rules whenever needed).

My guess is that you'll have a fight with AA to get them to refund your additional costs, so be prepared to complain to DOT and possibly your credit card when then time comes.

Is son seated in business class or coach on the segment that you upgraded? Are his seat assignments (which aren't guaranteed) worse than they were originally and did you pay extra for seats?

Is there any chance that your or his AA FF account has been frozen or terminated due to alleged credit card sign up bonus abuse? Anything that could be interpreted as the buying/selling/trading of miles and other AA benefits? Can you log into your and your son's FF accounts as usual and see itineraries, etc.? [There are two big and active threads here on FT, in the AA and Citi fora, about AA shutting down accounts in the last couple weeks and how to tell if your account is under review, etc.]

Assuming that nothing else is going on here, it's shameful that AA tried to downgrade and IDB your kid without compensation. Since seats were obviously still being sold by AA on the original itinerary, it's hard to see why the agent he spoke to at the airport couldn't simply restore his original flights and leave the original return flights valid as ticketed.