That won't happen. Even though it's still adequate for him it's very likely preforming under what a typical healthy kidney performs at. Long term slow rejection is nearly impossible to eliminate. No kidney transplant is forever and only in truly exceptional cases does it last the lifetime of the recipient.

> fascinating that the organ could live that long

All types of organs can live that long, there are 100 years old humans around. Probably many types of organs could live much longer, we don't know that, the humans having them die first.