Did the groomer use a dryer on him? That may have spooked him. The shop may have been a bit disturbing for a first visit.
Has he had his puppy fluff taken off yet. I used a Mars coat king on Bobby, which removed a cushionful. He was good with that, but wouldn’t let me do scissors. He was traumatised at one groomer, a busy place. Then by word of mouth I found a quiet,calm, confident lady, who had done courses on dog psychology. Difficult dogs were sent to her, she practically hypnotised them with her soft voice and stroking.
I’ve read that to start feet, preferably with a helper, lift one paw backwards and hold it while you scissor round the paw on the ground. When you want to trim under the paw, a groomer showed us a way to hold the leg still at the shoulder joint, then tip the paw backwards and trim with scissors. At home , my daughter just had to have OH pin her dog down on the grass!