You can interview trainers.
He may have a "tell" that you haven't picked up on. It's not common for a dog to be completely relaxed and then bite unless he is in pain. A trainer could probably help with this.
I actually think a muzzle might not be a bad idea, if he can be around other dogs with one on. Some of them are just a few straps, it's not like an umpire mask. But yeah, a trainer is a good idea. Please keep us updated.