I'd say wait for someone whose more experienced to give you an answer but as far as I know, once they reach adulthood, they can't heal deformities from molting. If his leg doesn't work, I'd say just keep an eye on for now to see if he uses it to balance even if he can't move it. Mantises like to knock themselves over when they grab stuff so it might be useful for his to have that support. If it actually gets in the way, then I'd say amputate the limb. Make sure he has a safe space to adjust too so he doesn't knock himself off something. Mine liked to bonk into the ceiling fan when I didn't keep an eye on him.
