I don't have any advice, but I feel so bad for him
I groom my own Yorkies for just that reason. Our Akita/German Shepard also got cut once by a groomer - which healed fine - but never took him back.
We were in the vets office when we got our 3rd baby, and this couple with their dog came in - so frantic. From what they said, Petsmart had cut their dog during grooming and the poor thing was bleeding in front of us. Petsmart paid for the vet emergency visit, but after that incident and our Akita/German Shepard getting cut, I won't let anyone else groom them.
Sorry you had to go through that.