It will be a recoverable overpayment if it isn’t your fault (whether you choose to pursue the o/p is another matter).
Does he act for himself as an HB claimant or does his mother claim as an appointee etc? If she does, that arguably puts her under a duty to disclose changes of circumstance as well (although even if she is not under such a duty, it still doesn’t make it the Council’s fault that the claimant cannot be relied on).
One advantage if the mother is acting for him is that you can freely discuss his claim with her, which means that you could remind her of the importance of reporting this particular change of circumstance each time it happens until the claimant is 25.