Sorry you are in this position.
First I would contact a lawyer.
Second If the dog has a bite history which this trainer and the rescue were aware of and never informed you I would look at turning the tables on the trainer if possible, see what the lawyer says. Just remember if he does report it to AC and the dog does in fact have a previous bite history this probably will not bode well for your dog.
Look into the laws as they pertain to dog trainers and bites, after all he is a trainer and trainers get bitten, they should have insurance to cover themselves for this.