I love being too academic!
What I was thinking, though, is that it can be helpful to take all emotion out of it, say it straight, say it like it's YOUR boundary, and don't leave a hook for him to grab onto. Because honestly, he sounds like the type who doesn't hear "no"--much like his kid. Maybe don't worry so much about how you say it, or about hurting his feelings--just be sure that you say it clearly, without leaving room for a negotiation--because I'll bet he'll try!