if you were holding an illegal gun or if he told you to halt and you took off running.
I don't know how they are teach to react in practice but if someone has a gun they can of course shoot that person. My guess is they would use a weapon on such a person. Would they use it right away, would they do it of that was a 13 years old boy. I don't know but the law says they can shoot if someone is a danger to life and health of a policeman or other people and refuse to listen to the police they can shoot.
What would he do if he ordered you out of your car and you refused
He wouldn't use a gun. That certain. Could use force but not a gun. Unless they suspect you were a criminal or you would have attempted to drive away.
What measures would he take to defend himself ?
He can defend himself but it wouldn't be his first instinct to use a gun. Against a knife or some kind of makeshift weapon yes but not against fists of a lone individual.