Look at dentalplans.com - it shows dental discount plans. He may take one of those.

Edited for more info...

The discount plans aren't insurance. A big reason some dentists don't take insurance is they have to hire someone to process all of that paperwork. With a discount plan the dentist just signs up to allow fixed prices for certain things. For example - they may make a crown $700 (no idea of pricing since it's been a while since I had one of those discount plans) where it may be $1400 normal price. The dentist gets more patients in, so more money for him, and doesn't need to deal with any insurance at all.

Look at dentalplans.com - it shows dental discount plans. He may take one of those.