"[At first], it seemed Blac Chyna was really encouraging Rob to stay healthy and clean," an insider tells PEOPLE of Rob's amped-up workouts and buttoned-up meal plan, changes he made after he started dating Blac Chyna, 27. "But her arrest is changing things. The drug possession is very concerning."
For more on Rob Kardashian's relationship with Blac Chyna, her arrest and how the Kardashian-Jenner family is reacting, pick up this week's issue of PEOPLE, on newsstands Friday
Now, Rob is no longer living with Khloé, 31, (who went "out of her way a million times to help him," says the insider) and has moved in with his new girlfriend.
His famous family, fiercely protective, is on high alert. Says the insider: "No one wants Rob to live with someone who does drugs. If [Blac Chyna] isn't helping him, Rob's family won't tolerate it."