Dear Scotty,
Understand that Hyundai are a bunch of scumbags.
First of all, never talk to dealers. They HATE warranty work and will do whatever possible to avoid taking it. Call national customer service instead.
If he has them, your son should get out his records and receipts showing he changed the oil regularly.
Try sending an email to Hyundai customer care explaining your situation and telling them your local dealer is refusing to replace the engine. See if they can put some pressure on this dealer. Hyundai knows very well about their engine issues and have replaced thousands of them.
My daughter's 2016 2.4L Sonata lost power on the highway 6 months ago @ around 60,000 miles and the dealer replaced the engine free of charge. He did ask me for oil change records however, which I did have. If you don't have any records, still email the Hyundai customer care and open the case, it won't hurt. Who knows, they still might replace it free of charge.