I have heard that there is a novel that follows the events of Mushoku Tensei starring the son of Rudeus but I don't know if it's true. Can you tell me if it exists and what is the name of the story?
Tl;DR: Yes. But the author doesn't view it as a "sequel" to Mushoku Tensei.
Rifujin na Magonote, the author of Mushoku Tensei revealed to have plans of writing a sequel that will be set eighty years after the Mushoku Tensei events. Magonoto won’t be writing the sequel story right away. As revealed in his Tweet, the author will wait for the whole Mushoku Tensei story to be animated, which will take around 3 to 8 years, before starting to write again. The new story won’t be focusing on Rudeus, but it will be set in the Mushoku Tensei fantasy world. Characters from the Greyrat family will play an important role in the sequel, but it is not known if they will be the main characters of the new story.
It would be the story of “Mushoku Tensei: Jobless Reincarnation” eighty years after the end. It will depend on how long until the end of the anime adaptation and serialization of the “Orc Eiyuu Monogatari: Sontaku Retsuden” light novels, maybe 3-8 years from now? Also, I do not intend to write this story as a sequel to “Mushoku Tensei: Jobless Reincarnation”