The only option you have is to give him the script and make him read from it. You owe it to the rest of your cast to make sure the play runs as smoothly as possible. No shame attached. Just tell him, "The show must go on," and it's now his job to read his lines from the script so the rest of the cast doesn't get screwed up. He's allowed to try to say them from memory, but he must have the script in hand. It's not about him - it's about doing what's best for the show, the audience, and the rest of the cast. Couch it in the gentlest terms possible, but give him the script to read from.