Hello blueeyes,
Former Marine here and I know some things have changed. Back in my time with the draft lottery and sometimes a felon entering about the only way out would be for a medical injury or psychiatric diag. I sure it happens on rare occasion but they'll also set people back for retraining or ship them off to motivation platoon to move mounds of sand. (an attitude check) If someone is having trouble on a force march they expect others in the platoon to get them through it.
My gut feeling is he had a personality flaw and was just not compatible in getting along with others. The Marines will get a person in shape, many hundreds and thousands of bends and thrust with pushups can be done in 13 weeks. I saw two dropouts due to medical reasons (a drill instructor injured a recruit by putting a boot in his back doing push-ups.
Times have changed though but I don't think too much.
Semper Fi