The best way is to have him professionally groomed. We see cats that have had "home" grooming jobs and their skin is cut. Some mats go to the skin and fester underneath. A professional groomer will be better able to take care of them. In the case of severe mats that are VERY close to the skin (where the skin is actually festering underneath), then they may need to be anesthetized and shaved by a vet. Try a groomer first - make an appointment for a lion cut shave and see if they can take care of it.
I HIGHLY recommend the lion cut! Go to a cat groomer. You want a groomer that specializes or is atleast comfortable with cats. I get the lion cut for my Persians every few months when/if they start matting. They LOVE it--they instantly become more snugglier and become much less stressed since they don't have to groom so much fur!
Also, my Persians fur ALWAYS grows back just as thick and luscious after their lion cuts. Some people will try to scare you out of it by saying it'll "damage" their coat, but it doesn't.
Not sure where you're located, but in my area, the lion cut for Persians is about $60. It is worth every penny to let the professionals handle it since cats have such thin skin! And it lasts for months before their fur grows back to the full length.
The only way to deal with mats is to prevent them. Unfortunately, I'm just not consistent with brushing so the lion cut is much easier and my cats love it.
Your veterinary clinic might be able to shave him if you can't find a groomer who can do it. When I had longhairs I used to get them shaved at the beginning of June, just before the hot weather started. We just took them to the vet clinic to get it done.