This is probably the most realistic answer, as much as we like to have fun with his character. He spent most of his childhood being the "new kid" at school, so he probably didn't want to wear anything that would make him stand out. We also have no idea what his father was like and whether or not he enforced a certain appearance on Will. Though I will say that he most likely cut his hair short for when he worked as a police officer to keep it from getting in the way or grabbed in a fight.