Before giving him sedatives for grooming I would continue trying. Working with rescue I've groomed many dogs that either weren't used to grooming or had been abused while grooming. It's not easy and takes lots of patience, consistent, and gentle handling.
I don't know exactly what you've done or used in the past but get a good grooming brush and comb. I use several brushes and combs during a grooming session but you can get away with one good comb and one good brush for now. Recommendations - Chris Christensen, pin brush or wooden pin brush, a good metal comb, mine is teflon coated bought from :
Dog Bows: Yorkie Bows, Maltese and Shih Tzu Show Dog Bows & Show Leads
Another vendor ( mimimomo) here on YT sells Madan brushes and you might prefer one of those brushes. she also offers combs but I don't know if she has the teflon kind like I linked to above. But, I would strongly suggest the teflon comb...mine has paid for itself many times over!
With dogs that fight grooming, I start out with them in my lap(sometimes sitting on the floor for better control). I talk to them with encouraging praise in a gentle voice while I brush/comb. Initially I make the sessions as short as possible. You can also try giving a treat while grooming. Try to make grooming sessions pleasant and fun, not frustrating and stressful.
It takes awhile to undo the damage someone has created, but he will learn to trust you, given enough time and patience from you.