I've had him since he was 2. He's 9 now. He's never let any of my family brush him. We've always had to take him to be groomed, but lately his hair got dirty from some chew treats. I decided he needed a bath because the chewing on the treats caused the hair in his mane to stick together. I was able to bathe him fine outside with the hose while holding his leash up so he couldn't snap back and bite me. I praised him while giving him his bath and gave him a few treats after. After I went on to brush him and he was not having it. He kept trying to bite so I got his muzzle, but he was still trying to snap back despite that the muzzle was on. I wasn't able to do all that needed to be done because of this behavior. I tried to cut his hair hanging on his sides and he still was trying to snap back. He's very sensitive and I don't know what to do to get him to allow me to groom him. I know he probably had a bad experience before I got him. Any advice?