In college, I had a motorcycle trip around the Great Lakes lined up with a buddy. I skipped out on a backpacking trip to Isle Royale with a few others to do it. A week before the trip, the buddy's girlfriend decided she didn't want him to go, and he decided to cancel.
I'm still bitter I didn't just go by myself, and wound up missing out on both trips. I'd go. Not your fault he failed his class, it is not fair to you to miss out on something you want to do because of his actions. Tell him you'll go next time with him, if your schedule and life allows it. If he expects you to be understanding, he should be as well.
Of course, as others have mentioned, don't bring it up until you actually know he failed, and that this is the only option.