I am thinking of red shirting a freshman with a 23 WE. I know if he isn't happy with it his WE will drop but at the start of next season will it be back to 23 or will it start at the level it drops to during the red shirt season?

it will not go back up. if hes 23 now, and you redshirt and it drops to 10, then it will be 10 to start next season. it will drop to 10 immediately though so you can try it and decide if its worth it - and if you take off the redshirt - he will guaranteed go back to 23. so its safe to try it, you can undo any damage safely, but if you choose to leave the redshirt on, any damage is permanent.