I would try Dr. Bach's Rescue Remedy. It is homeopathic. You can get it at human and animal health food stores.
Your dog has learned he can win the grooming battle, and it's not going to get any better without your help. When you try to calm him down and "talk gently" to him, I assume you're also petting him? Well, the talking and petting do nothing but reward the bad behavior. A good groomer who really knows dogs will understand that, so you'll probably need to change groomers until you find a good one. And no more rewarding bad behavior! :-)
Thanks for the recommendations. I will look into that. It may have to be another groomer as I did offer up the use of the muzzle at first and to bring him in myself to the back but she was not comfortable with that.
Suprisingly most of the time it's harder to groom a small dog than a big dog. lol. With big dogs the teeth are further away from their back ends, and their feet. It's easier to properly hold a big dog, and make them comfortable. We would trim and bathe dogs at the animal hospital in Florida I use to work for. There was one chihuaha that came monthly to get his nails clipped, and the kennel would always ask me to clip his nails while they held him. Poor baby was just scared. By grooming do you mean bathing and cutting nails, or trimming fur? If it's biting the groomers are complaining about than you might get your pup a muzzle just for the grooming. Muzzles do not hurt or effect their breathing. It will ensure the safety of both your pup, and the groomer. You could put it on yourself to make sure he is comfortable and it's not to tight or too loose before you take him inside. If I were you I would look around for other groomers though. We never had to sedate a dog to get him/her groomed or nails trimmed. And trust me I've dealt with some pretty mean dogs, and cats!