Let's not get sidetracked on the IDB issue as that has a specific meaning under US law and does not apply here, expressly because there was space.
When you call AA, don't be surprised if you can't get it straightened out over the phone, but inasmuch as the LAX agent did reissue his ticket, it should not be much of a problem to restore or reissue his return (without paying).
As to recouping the cost of the ticket, do that in writing and only after speaking with your son and obtaining an exact timeline, including a careful review by him of anything odd that may have happened before he boarded at SEA.