How long have you had him?
Is there anything loud going on outside your home or construction on the home?
I would suspect he got traumatized by the groomer then (not from anything wrong just the noise and fur loss etc) and may not like the sweater too.
Perhaps the last time he went to a new groomer he lost his home and so is being fearful due to bad associations.
If you have been trying to coax him and telling him 'that is OK' etc he may be responding to what he hears as your telling him he is right to be worried.
You might try the 'jolly routine' with him. You can read about this here
http://www.animalbehavior.net/LIBRARY/Canine/PPM/DogJollyRoutine.htm
its a way to change your response to the dog's behavior to get the response you want.
You may also want to read here about calming signals to help you communicate with your dog
http://www.diamondsintheruff.com/calmingsignals.html
Keep an eye on the dog's fur and skin though looking for hair loss, scabs or tenderness. Sometimes when shaving a dog the clipper gets to hot and the dog can get clipper burn which is uncomfortable for them and may make them feel 'off' and hide etc.
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics