The leg will regrow. He/she/it will be fine. As long as a mantis can hang upside down on its own without falling, it will be able to molt again and in my experience it doesn't matter which legs are lost. I've had nymphs missing two back legs, missing both right side legs, missing both front legs, and each of them has molted and recovered just fine.
One thing you can do is make sure he can easily climb back up to the top in case he does fall down. I like padding the sides of enclosures with paper towels for handicapped mantises, since it's a little harder for them to keep their grip on plastic with a couple missing legs. Something natural like a stick will make it easier for him to hold on as well. If you have his last molt date noted down, just keep a closer eye on him when his next molt is coming up. A lot of the times you can foretell when your mantis will molt by watching its abdomen. The closer it is to its molt, the slower their abdomen will become empty until it just stays full even if you don't feed. When you know he's going to molt, just make sure he's hanging upside down, or help him up if he's too lethargic because of his upcoming molt.
Hope I made sense, it's getting late x.x
here is a picture http://imgur.com/vIEZmSH
