How to get tough mats out?

Have a medium to long hair cat - 2 year old male.  I brush him several times a week (almost every day because he likes it).  The problem I'm having is that he has tough to get out mats in the armpit areas (I don't know what this is called on a cat, sorry!).  And he is sensitive to me brushing in these areas to get them out.  I swear he gets small mats daily that he is use to me brushing out but these other mats are just a pain.

Any suggestions?

A comb works better than a brush to get mats out. Use a fine toothed one. I think there are even combs specifically for mats. Take a look at what the pet store has. Just pick at a mat gently with the comb. For really stubborn mats you may have to carefully snip them out with a small pair of blunt tipped scissors.

Our longish haired male cat gets arm pit mats too both front and back.  Luckily his sister doesn't.  Be really careful with scissors.  I tried that and I actually cut his skin.  Luckily it healed right away.  I took him to a groomer and they shaved under his pits and tummy area. This was an all day thing and they insisted on bathing him before the shave.  Unfortunately I didn't learn my lesson and they came back.  The next time I took him to the Vet and they gave him a mild sedative and shaved his tummy and arm pits but no bathing.  I think this was less traumatic for him.  Now I brush him daily to include his pits.  He likes the brushing except for the pit area.  I save that for last.  I sounds like a lot of work but if done daily it really only takes a minute.  I keep the brush out where I can see it to remind me. 

A brush will not get the mats out. A comb (some people swear by the fulminator) must be used. A cat's skin is very thin and easy to accidentally cut. If a mat is right against the skin, let it grow out a bit before you try combing it out. If necessary, very small snips into the mat in the area furthest away from the cat's skin will help break up the mat for combing. It also helps to ensure that you are not cutting the skin.

For armpits, if you can get your cat on his back and stretch the arm (leg) towards the head, you will be able to access the area to be combed easily.

I should have been more clear when I said "brush".  It's a metal toothed brush that works very well to take out excess fur and tangles.

One of my Siberian cats is very prone to mats.   I had to cut several of them out with scissors.  This is risky business, but I did it in good light, lifted just a few hairs at a time with the scissors and only cut when I could visualize the skin.  It took a long time, like over an hour for a particularly big/noxious mat - it was as big as my hand!!  I was mortified that I hadn't spotted it sooner.

I also found that brushing does nothing to prevent mats because it doesn't get the undercoat hairs out.  Combing is better but it pulls on the fur and my cat hated that.  So I bought the furminator and he hasn't had a mat since.  About 10 seconds of quick swipes through the mat prone areas twice a week.  I also swipe his butt with it to minimize "cling-ons".  I don't use it on his back though, it really does cut the hairs something awful.  

Is he oily?! Does he go out doors?! I'd recommend several things as a groomer myself, if he's dirty he will Matt faster(example : greasy skin). However if he's pretty clean then brushing should be sufficient. You can if he's well behaved, regularly clipper just the sanitary areas. And that will help out a lot when brushing thoes areas brush a little then treats ! Make it a fun game for him, he probably doesn't like it cause he associates it with pain. Also really keep on top of it even if you just brush his armpits daily that will help a lot

We have a long haired princess, Belle, who I try to comb out at least once a week (her fur is finer, less dense than most). 

I also find the comb to be the best tool for the job.  It's easier to target a small area when needed.

For large mats, I do what I do with my own tangled hair;  hold it tightly at the base, and work it from the ends first. 

This is definitely trickier for the underarm area.  In that case, I actually hold the skin (not too pinchy), so the pull from the comb is less noticeable.  I work with just a few mats at a time, and always go back to areas that are more pleasant (Belle likes it on her cheeks) before letting her go.