They should have a small cloth muzzle they could put on him to prevent him from biting anyone during the shave, if that is their reason for sedation.
Gandalf is a long haired male and because he can no longer groom I have him shaved once a year. It is done by a clinic staff member who is an ex-groomer, the two of us do it together - I hold him for her. Yes, he gets mad, yes he swats and we put that type of cloth muzzle on him so he cannot bite.
Another reason for sedation would be for less stress on the cat. Next year I may ask them to sedate Gandalf because the procedure caused him to get extremely anxious since then when "it's time to go to the vet." I never tell him that, he just knows! After we got home that day he was giving me kisses because I think he felt so much better. Definitely get the mats shaved off - he will feel better.
Perhaps you could call a couple groomers and get their advice.
Fasting for 12 hours is mainly because they don't want any possibility of food coming back up while sedated. If the cat is not on insulin, it should not be a problem for him to be without food that long.
But sedation for a shave should only be necessary in extreme cases of anxiety or aggressiveness.