Your best bet is to break the tickets up on multiple reservations. Assuming one way is 1/2 the round trip price, put 7k amex on your sons account and buy a one way. Then 1k on your wife’s and buy 3 one ways. And put the remaining Amex points on yours and buy 4 one ways.

Of course the math may not be this neat, but you should get the idea.

Finally, to make checking in easiest, have your sons account buy a grown up ticket and keep an adult and two kids on one reservation. Otherwise you’ll have to check in the kid at the counter on the day of departure to prove they aren’t an unaccompanied minor.