If he has not "shipped off" yet, then he can simply refuse to go.
Under the rules, he is not subject to the Uniform Code of Military Justice (UCMJ) until he actually goes on active duty. The is the "shipping off"
So if his date to report for active duty has not yet arrived?
He can simply refuse to go.
Since he is not on active duty currently, they can not prosecute him under military law.
They will give him an "entry level separation
". This will not impact his ability to go to grad school, or to carry on with his life.
Now..if he actually goes to bootcamp? Then it gets much more complex...once he falls under the UCMJ and can be prosecuted if he refuses to follow orders.
But if he has not yet left for bootcamp? THen it is a non issue...he can simply refuse to show up.
All they will do at that point is separate him.
He will not be able to join the military ever again...but it will not impact him otherwise