When should I give my pet stress tablet?

You may want to look into going to your vet's office if your dog is matted. Matted fur can be really painful and sometimes the skin underneath can have sores or other problems that need to be addressed. Your vet can give your dog adequate sedation, sometimes an oral medication or sometimes even an injectable sedation that allows them to be asleep, while they are getting shaved. This can help with the anxiety of the actual grooming process, which can be long if he's a larger dog or severely matted, and help any physical discomfort he may experience.

I'm thinking that any sort of sedative you would be able to give him yourself wouldn't be an effective solution. Some types can lower a dog's inhibition, so that not only is he still upset, but also now he's more likely to act out in an aggressive manner. In my experience, what would be the least stressful for him would be to be sedated at a vet clinic and have a shave-down.

I'm guessing he is also difficult for you to brush at home. Try slowly training him to view grooming as a fun (or at least, not horrible :-D) experience. Start with giving praise and treats when the brush is just laying near him. If he's okay with that, pick up the brush, but don't use it on him. Then let him sniff the brush. Keep taking very small steps. Touch the brush to him very lightly for a second. Move the brush along a small portion of his back. Very gradually increase the amount of fur you brush, all the while praising and treating. Keep each training session extremely short. For example, take the brush out of its storage place, put it on the ground treat and praise for 5 seconds or so, then put the brush away. Do several sessions throughout the day, and gradually work your way up (over a period of weeks, maybe) to teaching him to be okay with being brushed. I would also recommend making sure the brush is a type that won't scratch his skin. You may even want to use a comb instead.