grooming

Yes it is possible to sedate for grooming. But a few questions might also give you some answers.
How old? How frequently does he get cut--(before the problems started)?
 

polo is three years old.  He used to get groomed every other month until recently.

It could be that he had a few bad experiences either with you or with the groomer.  When you are grooming is he in any sort of discomfort, do you shave him down yourself or are you trying to brush out mats? brushing out mats can hurt, so can a hot blade, if your grooming and the blade starts to get warm take it off immedietly...clipper burn is painful.  Does your pup have any skin problems, redness on the skin. etc.
 
I would make sure all grooming experinces are positive.  If your current groomer is busy then try another groomer with a slower pace.  My Sister sets aside one day a week for her special needs grooms, she has also rehabilitated many dogs that hated being groomed...now they are perfect.  She has never had a dog sedated to get groomed...and she has rehabilitated some aggressive dogs including large breed dogs.
 
Have the groomer to show you how to properly brush out without snagging on any snarls or mats.  I wouldn't resort to sedating until you try brushing properly, and learning to use clippers properly.  A few bad experiences can make a dog offensive.

Dave I believe you mean Shih Tzu mix.  Its not a curse word named dog lol.  Harley gets matts easily.  He gets bathed once a week and brushed daily.  I clip him myself.  I also use coat conditioner on his hair which helps.  You need to find another groomer if the dog had a bad experience with that groomer.  Was it the same groomer as always or a different one?  Ask around and see where other people go and what their opinions of those groomers are.  I think sedation should be a last ditch effort.  Are you sure the dog isn't experiencing any physical pain?

Bathing once a week keeps the hair clean, mat free. Use a great shampoo and conditioner for dogs. I use Solid Gold.
 
I groom Trudy myself so I can do a little or all at once. Give treats when you do a bit of brushing. Then leave more for later.
 
She probably had some pain and I do not blame her. Sometimes they get hurt when at a groomer. Happened to Trudy and  now I have groomed her for years.