hi XXXXXXXXX,
I wouldn't think I would put him to sleep just because of matting. You can call other groomers in your area and see if they would even shave a cat, especially a matted cat and get their prices. If he is too matted, it is easier on him and the groomer to shave when he is sedated. Then just shave down his entire body every 3 to 4 months to prevent it from getting out of hand. It is not uncommon to get bald spots when they are matted. Usually the cat has bitten out some mats on his own and left them.
The other option is buy a pair of electric hair clippers and try to do it yourself, but clipping a cat is not the easiest thing to do at home. I recommend trimming the toe nails first and make it a 2 to 3 person job if you haven't tried it before. It won't be the prettiest of jobs, but he may be more comfortable. You can also do a little and then let him and you rest for a while.
But if you think his quality of life is suffering for other reasons then it might be time to have him euthanized. Only you know how he is feeling in his day to day activities.
Good luck,
Dr. Candy
Cat Veterinarian
7+ years in general veterinary medicine