Beware of in any way suggesting that he has a medical need for J. The result will be that he won't be permitted to fly in Y and you are then stuck with either purchasing the J ticket or not flying. A little bit of swelling at ground level is an annoyance. At 40,000 with a medical diversion airport 90 minutes away, it can be life-threatening.

Air carriers don't take chances and once you've told them he needs it, it's all over.

As to simply asking whether they might consider it just as a matter of comfort and not for any medical reason, the answer will uniformly be no.

In no way suggesting that your son is faking it. Simply that if it worked for him, there would be 1,000 people showing up on crutches every day.