First of all, congratulations on directing your first play--you're near the end and that's exciting.
At this point (with a week and a half to go, yikes), I don't think it will be beneficial to let him screw up his lines during rehearsal for the sake of teaching a lesson--that's not fair to the other actors, and would probably end up hurting the play. I would, as others have suggested, sit him down and tell him, firmly but kindly, that waiting so long to get off-book is irresponsible and virtually unheard-of in the professional world. Tell him that you expect him to be fully off-book by the next rehearsal. That's always worked for me in the past--and if he needs them, you can give him some tips for learning lines. If he has long monologues, a good trick I've used is recording them (without inflection) and listening over and over as I perform some kind of menial task.
As for the others, simply let them know that you appreciate their commitment to the show and the work they've done, both in rehearsal and just by knowing their lines. This might also help to shame the fourth guy into learning his. They shouldn't have to deal with this situation, but it's also not your fault, so don't feel like you have to appease them.
And finally, as for opening night . . . just know that it's out of your hands. For the last show I opened (back in November), I was an absolute nervous wreck, and I don't know that there was any real way of avoiding it. But if you have friends/teachers/critics in the audience, don't sit by them. The important thing is that you put on an excited face for the cast, and make them believe that everything is going to be fantastic.
My current show opens tonight. Our last dress rehearsal yesterday had to be canceled due to illness. I'm pretty worried about it. But by now, there's nothing I can do, so I'll simply show up to the theater with a huge smile on my face and enjoy the show, for once, without the stress of trying to improve it.
Good luck! I'm sure everything will be fine for you. And when your show goes well, there's really no other feeling like it--so enjoy!
I chose, and my world was shaken. So what?
The choice may have been mistaken. The choosing was not.