18X is not an actual MOS.
Tell him not to worry about it. The majority of those that attend Special Forces training rarely make it past the first initial steps .. and these are people that truly want to be there. If it is not in your fiance's heart and mind, he will not pass and will be sent off to a conventional unit .. most likely the 82nd Abn.
No one is going to force him to become a Special Forces soldier .. in fact, no one can. He has to want that himself. If he has changed his mind, then it's up to him whether or not he succeeds in becoming one. He doesn't need anyone to change his contract or MOS - all he has to do is quit.
Once he arrives as SFAS, have him inform them that he does not want to be there. I can assure you, SF does not take those that are not willing to sacrifice everything to be there .. they will not force him to stay. The same thing goes for the Abn training (BAC) he will be required to attend.
If he can handle being a quitter, then his problems are solved.