Yes catch him and, GENTLY, pull the lip forward, pressing the bone (The long skinny one that is most obvious) into alignment with the head. The guide.push, GENTLY, the bone and mouth into the proper closed position.
If you leave him like that for too long his tendon will heal and, usually stay like that irreparably. It's not that big of deal but it does look weird. If you can get it back in place you should separate him into his own hospital tank, feed only every 2-3 days for a couple weeks, feed small pellets or flakes (So he doesn't have to open his mouth all the way to get whatever it is). The idea is to give him less reasons to extend that joint until it heals. That way, you have less of a chance that the tendon will be destroyed by repeated use or micro-trauma.
I know it's just a fish but they're fun to fart around with, especially when it helps them out. I have a friend who is an M.D. and a Tang keeper and this happened to a big Calvus of his... So using a butterfly (very Small) suture needle and a human hair we applied "Kanka" as a numbing agent (Mostly as a joke/experiment because "Kanka" contains a gel which hardens like a layer of skin once it's wet. It also contains a bit of Benzocaine for numbing) and then performed a single "loop" to knot suture (one hole) from the cartiledge the very tip of the nose, extended the loop over the mouth and knotted it off. Effectively wiring the upper jaw shut. He ate and healed just fine as his mouth could still open and shut, he just couldn't "suck" food up like what is necessary when they're hunting live food.
I'm almost embarassed that we did that but, wether or not that did it, that fish has not had the same problem again.
We were very particular about using the smallest of needles and the hair was also because it was very fine and smooth. We were planning to remove the suture but after 3 weeks, the hair grew brittle and the Calvus shook it out. It cracks me up to think what his patients thought about this fish with stitches in the waiting room!