Yeah, don't suggest he volunteer anywhere!
First, be really, really slow to get back to him--you're busy!
Then, say, "that was interesting, but I really don't have time to meet. We've covered all of these points in our research already, so it would be a waste of my time and yours." (remember, you need not tell the truth at all--even if he thought of something you didn't, SAY THIS. Because, you are establishing that you and everyone else are far ahead of him)
"Thanks for thinking of the cause, though! Hope to see you at one of our fundraisers, like the barbecue." (Pick something he has to pay admission to get into.)
Then, do the fade-out. Never reply right away, and sometimes never reply at all. If you do reply, say, "Sorry to be hard to reach--we're pretty busy. I don't have time to meet. See you at the silent auction!"