How do I handle a supervisor who doesn't want to work?

Let me define supervisor. He's not necessarily a trusted employee who rose through the ranks. He's definitely not a manager. I just mean he's certified to work on cars and I'm not, yet, because I'm new. So... he supervises me.

In the last three hours of work, the only people there are me and him. I'm all for living and let live but he doesn't do his job and when everyone comes back in the morning, I'm sure they're saying, "M and B didn't finish their job." (M being me and B being him.)

I'm the exact opposite of him. I like to make sure everything is done and then some.

He does things like closes the shop early so he can leave before he's supposed to. I feel like I have no choice but to HELP him close the shop early because I'm not qualified to work on cars. If he leaves, I'm not allowed to do anything. And I don't want customers coming through the open doors and expecting me to help them, just for me to say "I can't".

So I'm just kind of funneled into this "M and B closed the shop early because they don't want to work" reputation.

I don't know how to handle it.