Don't use a scissor on those clumps unless you want a trip to the emergency vet for stitches! Cat skin is EXTREMELY easy to cut. And when the fur mats it pulls the skin up in to the mat. That is another reason it is so easy to cut the skin. First suggestion....clip the nails BEFORE you try to brush. Next suggestion.......do only a little at a time, this way he knows it won't be a long session. If you stop BEFORE he gets upset he may learn that it is not such a bad experience. If it is more than you can handle have the cat professionally groomed this once and then keep up with him so he does not mat again. PREVENTION is the key.