My son quit selection. What job field would you feel would be best to try to reclassify so he can come back and try Selection again?

As for him quitting, he will most likely have another chance if he were to reclassify and come back in a couple years. From my experience as an instructor, the younger candidates are more prone to making impulsive decisions and I have seen it time and time again when the student will have a moment of weakness. I always tell the students who quit that it does not define the person you are if you don’t let it. Take the time you need to get over it, then learn from it and get back on the horse. If he has the option I would recommend him to try out for SERE then go back to PJ when he is ready.