As the others said he can back out. It would be most mature to let his recruiter or the recruiting commander know about it. Considering he is going to school and the military is overmanned I doubt the recruiters will care too much. Marines are meeting their recruiting goals easily and are over strength as well.
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"The American people will never knowingly adopt socialism, but under the name of liberalism they will adopt every fragment of the socialist program until one day America will be a socialist nation without ever knowing how it happened." – Norman Thomas, American socialist