Simple solution.
1. Check your server logs, and add up all of your downtime.
2. Calculate exactly how much $ you should take off of what he owes you.
3. Offer him the discount, and mention that you can prove the downtime.
4. If he doesn't bite, let him know it's a last ditch effort before you go public on the boards with the issue.
5. If he still resists, post his name.
6. Send him the URL to this thread.
