I'm not sure the IRS is even allowed to call you. They have to communicate through mail. So if anyone claiming to be from the IRS calls you I would demand proof, like an ID number of the person you're talking to.
You said he had a beer belly. He was probably drunk. You also said you paused to see if he would cut you off and go first through the drive through line but he stopped moving. My feeling is he's a loose cannon and deliberately did what he did on purpose so he could get his jollies out of intimidating a woman. He's probably not had sex in years and hates all women and waited till he saw a woman go through the line and planned the whole thing. I still wish you had taken down his license plate number.