To be fair, there's nowt the club can do because he is still on a YTS contract and, I believe, has rejected a first-team deal because if he signs a pre-contract agreement, a tribunal sets the fee because of his age, and the tribunal will set the fee based on his current salary.
So, aye, Celtic are getting to steal a prospect for peanuts again but this time it is the fault of the system and the player/his dad.