How do I tell my boss I can’t make the shift this week?

I'm a programmer, and every week, a person does shift 24/7.

We go to work like any other day, but after work, if something happens, I get a phonecall (often in the middle of the night) to resolve the issue (monitoring servers or databases)

We already know which person comes after another, so I make sure not to make plans on the week I need to work. Still, sometimes people change their shifts because they will take a vacation or want to be free to go to concerts, so someone does shift earlier than expected, which is annoying because they have to cancel plans. So a few days ago, I asked my boss which people will work this week. He said Jim was, and then me, but of course, he forgot, and now he is asking me if I am doing shift this week, which I don't want for these reasons:

1. Jim is available but doesn't want to because he needs the credentials to the servers; he didn't ask for them.
2. My boss forgot Jim was next.
3. I wanted to rest this weekend.

Apart from this, there is another issue about risking lives because he encourages my co-workers to learn to drive on highways, but that's another post.

Any suggestions?

Thank you.