I second Rebecca's response about modeling the behavior you want to see. When you take him out to the bus, you should wear a hat, too, and make a big deal out of it. I have a 2 year old with developmental delays, and last winter when he couldn't even walk or talk yet, he picked up from my husband that he had to have a jacket and hat on before he went outside. Daddy always wore a hat so my son never questioned it. He wanted to go outside so badly that he would do anything that was part of the routine of getting ready to go out.
This went on all summer - we finally broke him of having to have a jacket on, but he still wanted a hat. And now that it is cold again, he is back to a jacket and hat. He loves hats so much he usually even wears his hat in the car on the way to school / day care. He has his moments when he doesn't want it on and will throw a fit, but it is just him being a 2-year-old. We make sure that we are still the boss and he wears his hat if he needs it. Sometimes having a CHOICE of which hat helps. Present it like it isn't "hat or no hat", it is "the red hat or the blue one".
Good luck - loving school and the bus will hopefully be a good incentive. I know my son will do things for his physical therapist that he won't do for me, so maybe yours has a favorite teacher that he will want to impress by wearing a hat?