Technified Ex's statements have legal merit.
The general rule when it comes to employment contracts is that they are presumed to be valid and its terms and conditions contained therein are the embodiment of the parties' intentions when they signed such document. Hence, such contract is deemed binding to both parties.
Unless there is sufficient proof that willful fraud or misrepresentation has been committed by a party or if the terms and conditions weren't sufficiently negotiated by the other party (a contract of adhesion - meaning, only the employer exercised the power to negotiate and the employee was merely made to sign without understanding the consequences laid therein) or if such a contract is contrary to law and morals, then the presumption of the validity of the contract subsists.
Technified Ex's suggestion on how to deal with the current dilemma is what I would advise you to do as well, hoping that your son's current employer is understanding enough to let him go of such contract.