His family would have told him about his grandfather returning to the light, or Anakin’s force ghost could have told him.
That’s another thing about the new Canon that bugs me. Apparently, Luke, Han, and Leia all kept the identity of Ben’s grandfather a secret from him, and he only found out about it through the Holonet. I think that’s very out of character for all three of them. Realistically, Day 1 Lesson 1 of Ben’s Jedi training would be a Skywalker family history lesson, so that he could learn the cautionary tale of Anakin.