My Siamese is thin? Is something wrong?

You should do a senior blood work up on him. Also pay attention to how much he is drinking does it seem like more than normal? Your vet will want to know. The only way to know what is going on is from blood work. Blood work could show thyroid or kidney problems, but you are on top of it and there are many medications and special foods that can help and in some cases reverse what is going on.