Sadly, we do have some volunteer opportunities that would suit his interests and abilities, but he is no longer interested in pursuing them. I am speculating, but I think with the taste of administration he got before, he now believes that this is his calling with our program. Unfortunately, we don't need administrative volunteers. We have a whole board of directors for that.
I agree with a lot of the previous posters, but I would like to suggest some wording. I think it might work well to say (or write) something like "What we need help right now with is..." or "Volunteer positions currently available are...".
If you send him an email or letter, it could say, for example:
Thank you for your interest in [thing that needs volunteers]. We are still looking for help with tasks ABC, XYZ, and QPL.
Please let me know if you are interested in any of those positions.
The point being, basically, to say that this is what we need and we don't need anything else. That saves you from having to directly say "we don't need your help" even though that's what you mean regarding particular tasks he wants to do. He might hear "we don't need your help at all and we don't want you to play any role whatsoever in this", which would really insult him and doesn't sound like what you mean anyway.