Define "dead awful" at practice...
I was always under the impression practice was the perfect time to experiment, to screw up, see what works and what doesn't. Better there than on stage, right? And if he's nailing it on stage then that tells me (from what little I know) that he knows the show is not the time to go screwing off and playing around with things he shouldn't be playing with.
+1
Also, not sure how long he has been with your band or how often you have practiced. But IME, some guys take awhile to feel comfortable with a new group, might take him a little while to settle in and feel comfortable to let loose.
I'd say if he nailed the gig then give him more time, and see if he keeps doing well at gigs, if he does then there should be no problem.
Now if you do have choreographed parts you are going to have to work on them at some point, same with lighting plots, pyro (don't ask me how I know the importance of this one. Do they still sell flash-powder? New eyebrows?
Just sing the dang song Jethro.