If they want to pay for it, maybe ... if there's a particularly interesting or important commencement speaker. otherwise, how about throwing a nice party and inviting them, the day of his graduation. That way it will be an "event" and your in-laws can tell everyone they went to their grandson;s graduation.

It's one day, a few hours, and shouldn't be a huge deal either way. Usually you don't even get your actual degree that day, just a ceremonial piece of paper that you exchange for the real thing later.