Good question. Have you talked to anyone from Apple or gone to your local Apple store? They might be able to help, since it is a Apple ID issue. Also, if it isn’t the same Apple ID, I don’t think it will transfer, but I could be wrong…
We appreciate your long time business and purchases. Unfortunately I do not have the ability to transfer your IAPs to the new ID, as Apple handles all transactions.
Contact their support team, explain the situation and request that the Infinite Flight specific purchases are transferred to your son’s ID. As the account owner you can authorize this. I appreciate you’re understanding and don’t hesitate to contact me if I can help in any other way.
It should be fine if you sign in with yours and get the aircraft then sign back out again. I use my mothers to get all my IF IAP’s then sign back out but the family sharing may mess it up, haven’t used it before as it seems pretty useless.