Army prior service question...?

My husband is in the National Guard & wants to go active duty. I know with the Prior Service Business Rules that are out right now, he can't. His MOS isn't available. We've heard his MOS might be available in Oct. when the new fiscal year starts & we were thinking he should start getting his conditional release filled out (that'll take a while...) so he can be prepared if a slot becomes available. My question is, if he gets his conditional release & goes to MEPS, but his MOS isn't available, does he HAVE to pick another MOS to retrain to? Or can he just go back to his unit? He doesn't mind picking another, he just doesn't want to get stuck with something he hates. I tried asking on the Army Recruitment facebook page, but they just gave me the run-around (big surprise, huh?). Any RELEVANT information is highly appreciated. :)