The neck line

After an incident in Afghanistan, a mission gone wrong, Tyson doesn't wear anything with too tight of a collar.
The clothes that he wears are either very loose and bellow his collar bones [t-shirts and polos] or stop on the collar bone and are tight enough that they can't slip any higher [basically anything else other than dress shirts and formal dress shirts*].
When he wears a dress shirt he doesn't do up the top button and lets the tie or bow tie [or any other type of fabric worn around the neck] keep the collar closed. When he wears a formal dress shirt he'll close the top button but keep the neck wear loose to avoid triggering any flash backs. 

*Having come from an upper class family Tyson considers anything worn on the torso that isn't a vest or item of clothing worn over a shirt [blazer, jacket, coat, et cetera] to be a dress shirt. What most people would normally call a dress shirt Tyson calls a formal dress shirt.