Sounds like you need a new groomer. I personally have a bad experience with Petsmart and would not recommend them to anyone~ever.
Maybe you could try calling your vet to see if there is a local private groomer that would be willing to take their time with your little guy. Sounds like he might have been traumatized in the past. I would not recommend the use of tranquilizers for a grooming.
When I was looking for a new groomer I called my vet they gave me several people to call and I interviewed them on the phone and then took one of mine along to meet her. I also had my foster at the time and he was a handful for the previous groomer and they would upcharge me. My groomer now only does my pups and never overbooks. She will take all 3 at once and take her time and not rush. The poor little guy needs to be worked with slowly and with lots of love and care.