Have Military Law Questions? Ask a Military Lawyer.
The national guard and reserve recruits are in a different situation than the active duty recruits (who can opt out before leaving for basic training).
They are considered sworn in the first time they do so, and so he is now already a service member under a contractual obligation. Once your permission is given and he has signed up, he is in the service. You can't retroactively revoke the permission you already gave, because the contract has already been legally entered into.
Now, you can certainly contact the commander of the unit that he is in and work to see if that commander will separate him based on a failure to adapt, hardship or some other reason deemed appropriate by that commander, but it will be that commander's discretion. There is no legal loop hole or right to a separation that he can push on the commander.
If the commander is not willing to release him, then he must go to basic training or he will be listed at AWOL. That could result in a warrant for his arrest or in an administrative separation from the service with a punitive discharge. While not as bad as a criminal conviction, a punitive discharge can have lasting effects on a person's civilian career options.