4. He is put into hiding again
If Grogu is still in his toddler stage, even after training with Luke Skywalker for many years, he could still be protected much like the way he was when he was out hanging with Din Djarin. If Luke becomes as protective of his little green Padawan as Mando was, then he may have looked to put him into hiding when he sensed the growing darkness in Ben Solo.
If Luke sensed that Grogu could be in jeopardy, he could have contacted Mando to come and take him away. This could have been done in secret with only Luke knowing where exactly Grogu was going. Mando, even at a much older age, would jump at the chance to be reunited with what was basically his son and ensure his safety.
Luke could do this with the intent that he will continue Grogu’s training after seeing how the future unfolds. After his Jedi Temple is destroyed and he denounces the Jedi ways, he decides to not contact Mando and leaves Grogu’s ultimate fate with him. Grogu would then stay with Mando and could either live the life of a Mandalorian or return to the Jedi Order and continue his training with Rey.