I'd go to Mendoza if you're okay with your son trying some amazing reds. Best option is probably LAN (or AR), since you only have 2 days. There's a nice PH over there, and lots of places to visit. If you won't be coming back to Argentina in quite some time, then do it. Otherwise, if you know you'll be back.. I'd suggest you stay in Buenos Aires and get to know it well. There's lots of things to do in here!