In addition, he was just recently told

"It is clear, from the stories, that the Dark Lords who return by possessing another's form, wield lesser magics than they once knew. I do not think Voldemort would be satisfied with that. He would take some other avenue to life. But Voldemort was more Slytherin than Salazar, grasping at every opportunity. He would use his pitiful state, use his power of possession, if he had reason. If he could benefit by another's... inexplicable fury." Albus's voice had fallen to almost a whisper. "That is what I suspect happened to Miss Granger."

So now that he knows it's theoretically possible...

Edit: I'm an idiot. He's known it was possible since Quirrell told him to pretend he was possessed by Voldemort in TSPE.