I believe they can withdraw anytime up to Jan 15th which is the deadline to declare- lists get sent to NFL teams Jan 19thI don’t find anything that says a player can withdraw. Last year there was apparently a 72 hour window to change your mind, but I don’t see anything about that for this year.
The surprise about IW is this isn’t like basketball there really aren’t other options to play professionally if you don’t make an NFL team and he most likely won’t- USFL pays 74K a year for reference. I would think making NIL in college one more year would be very good for him