when he asks you for a lift just say "sorry mate can't" don't say anything else or go making up reasons, continue about your business and don't feel you have to make conversation, if he was a friend you'd owe him an excuse ....
if he's a bit thick and doesn't cop it first time just keep saying the same thing... don't elaborate, he'll eventually get the message...
sometimes you gotta stand your ground and put yourself first...