UGH!!! I have the same situation and my cat is not even Siamese. He gets matted in areas where the hair grows longer, like under the paws and belly, and his pantaloons. I brush and then another one appears the next day. I have taken him to the vet for shaving but the hair grows back so fast I can't keep up. It hurts him when I try to get rid of the matts with the brush, and that's understandable...I'm pulling hair. I always end up hurting the cat. So yeah...shave him if you have no other choice because I am about to do the same thing....just get shave him completely.
Extensive fur matting is VERY common with Siamese cats. They require pretty constant grooming to avoid it, I don't think there is any real way around it. If he's currently very matted, you actually should have him shaved or clipped as it is very painful to their sensitive skin to try and unmat seriously matted fur. Then, as it grows back, just keep at it - lots and lots of fur grooming. If their fur isn't matted and you are gentle, it should be enjoyable for both of you. You can go ahead and have their fur clipped short periodically to make it easier on both of you. If you have a groomer show you how, and you are careful, you could possibly do it yourself, if you invest in the proper tools.
I have never used the Furminator products but, by all means, give it a try. From what I see, it could be useful. But you'll still need to put in the time and effort.
Does his fur seem dry? If so, it could be making the matting problem worse than it has to be, and should be easily corrected with diet, supplements and topical products. Also, is he shedding excessively? If he is, it might be something to get checked out with your vet as extreme shedding is a symptom of some health conditions.
Good luck! I know it can be a frustrating problem.