Offer to help. Don't put him on the spot. Speak softly, calmly and assure him everything's going to work out beautifully. Remind him that you've done this a thousand times before. Establish the rules, with a smile.
Deadlines are real.
So are the overall objectives of the web site. And so is your audience. These comprise the rules of the web design dance. Content needs to be on time, on topic, and appropriate to the audience. Know when to call the bouncer over.