If he has feathers on his stomach then he will start to feather out all over. I find the longer birds are inside the slower their feathers grow in. I used to think it was the amount of heat they got inside which honestly I think is partially true but now I also think the sun has a part to play even if it’s colder out. So now birds go out just as soon as feathers are mostly in. What are the daytime /nighttime temps where you are right now?