I'll assume that either you or the band is "producing" and replacing the drummer all togther is outta' the question right? I've actually had some A&R mooks request that the drummer was replaced...but that doesn't sound like this the case here.
That's kinda cooler...its the way things USED to be done!
If that's the case, don't even bring up the subject of using a ringer unless 'ya want to take a chance on getting fired!
OK.
You'll have to isolate all the guitar amps & stuff so that the
only "live" instrument in the room is the drums. Set up headphones & let the band track in the same room with the drummer so they all have visual contact with each other for cues & stuff.
If the drummer can play to a click, then do that...it'll make it WAY easier to cut takes together.
If the drummer
can't play to a click, then at the VERY least you can use the program of starting each take with a click & turning it off after the first 8 or 16 bars. That'll at least give them the same starting tempo on each take so with any luck they might be at similar tempos & be slightly easier to cut takes together.
If he's lacking consistency in his hitting you might want to stick triggers on the kick & snare and print those down just in case you need 'em later on.
Selecting the "right" cymbals, snares & tuning the rest of the kit is pretty paramount too. With the right selection & mic choice/placement coupled with being diligent in tuning & selecting cymbals that balance out with the acoustic volume of the drums you can get things to swing towards your side & make the drummer sound "better" then he is.
Not an easy task & if the drummer has lost his "R" (dummer

) then you might be screwed not matter what you try to do...but you'll need to invest some time into it.
See if you can talk to the band memebers too & see if they can get on him in practice to break some of the bad habits. I've seen HUGE improvements in drummers over the course of a month or so if they know they're gig is on the line.