Just a friendly suggestion here but, I would probably avoid the petting and hugging while he is frightened because this is just nurturing the frightened behavior, he needs to work through this himself without the human telling him it's ok...I would hug, pet and treat after he has calmed down...What you may want to suggest to your groomer if they are willing to work with you is clipping one nail at a time, wait until he calms down and then reward his calm behavior, and then just repeat this until it becomes second nature to him to have his nails trimmed....Gaby was also afraid when I first started trimming her nails and this is the procedure I used and now she lets me trim them no problem...
good luck!