i imagine that Kenny wont be able to regenerate after his mom dies. One day he will clumsily let himself die without even considering the possibility that his mom had just died too, and he wont come back. Karen will have lost both her big brother and her mom in the same day.
I don’t think he is truly immortal, Kenny CAN die, he dies all the time. He is just resurrected after dying through a past satanic ritual cause of some sort. It’s different than how Cthulhu isn’t able to actually die.
