It's obvious how much you love your cats, and they're both beautiful.
Rather than buying yet another grooming product, I would suggest you find a groomer that will take cats and get him a lion cut. As docile as he is, I don't believe he would need to be medicated. I can hold my cats while the groomer shaves them.
Then you can go from there. As much as he likes getting brushed, you should be able to keep the matts under control for the most part. Cinderella could get a matt 10 minutes after being groomed, drove me nuts! You're absolutely correct, he is in pain and needs help asap. Do you have a vet that could recommend someone? My vet has a groomer on staff, maybe they could give you a break on the cost.
Welcome to the forum! I'm so sorry about your boy, I saw the whole video. You're such a caring dad for these kitties. What a huge mat!
I can't offer you any help or advice, just what Marie says that sounds like the only option now: lion cut and start from there. It doesn't seem there's anything you can do about that huge mat with the brushes, least of all trying to cut it yourself that could be really really dangerous.
Good luck and the best of wishes for you and your kitties!
I'm coming in late tot this thread, so I hope you have already found a solution to your cats issue. I have a orange long hair tabby who gets small ones on his belly and in his leg pits too. Brushing him doesn't keep them away either; thinking of a shave job. Not crazy about the lion cut, looking for a "short hair" look.