What can I do about my siamese cats constant howling?

I think he needs some new, novel toys to keep his mind occupied. Try a Peek-n-Play toy. I used a cat psychologist to help me with my Siamese and her pricey advice was right on accurate and so I will p it along to you for free! In the peek-n-play toy (try searching for one on ebay) put treats or small toys- they have to work to get them out and that's all part of the prey sequence that cats crave apparently. If that doesn't work, try some calming spray, Feliway, its a calming mist that you spray and it is full of hormones or pheromones. They also have plug in units of this stuff that work well too. If you use both of these methods, I know you will see some real results!!