Theory time: I think this new mysterious Saiyan is the son of the first original SSG that appeared on Planet Vegeta. Minus(which probably isn't canon) mentioned that Frieza has heard about the SSG meaning it must have shown up before but likely after Yamoshi who was the first SS. Yamoshi's spirit which wandered through time found 6 pure-hearted Saiyans who were opposing King Vegeta's rule and so fought the king and his servants which had this special Saiyan(M20's father) accidentally awaken to SSG thanks to Yamoshi's spirit helping him attain it but because this Saiyan couldn't handle the power, it went away quickly and he was killed by King Vegeta and the rest of the Saiyans as Great Apes. But his unborn son would eventually be born and sent away to some distant star like Goku before Frieza nuked Planet Vegeta. Frieza may know M20's father but may not have known that he was the first SSG. But he may know that King Vegeta and other Saiyans like Bardock were responsible for M20's father's death.
If M20 was out training on some distant star near the end of the planet and probably sensed Goku's SSG power, then maybe he and Frieza(who is off scouting the universe) bump into each other and Frieza recognizes him through his unique golden eyes that no Saiyan had except M20's father so he deduces that M20 is his son. Frieza then tells M20 about the source of the SSG power M20 sensed and tell him there are other Saiyans out there. Then he tells him the truth about his father and what the Saiyans did which prompts M20 to head out to Earth to confront Goku and Vegeta.