Have you seen him grooming himself more than usual? I've had a few cats over the years, over groom themselves due to stress. They tend to focus on one area of their body. It's not really noticeable right away because we are used to cats grooming themselves often, and you don't always see more hair.

The stress factor can be something really minor such as a new noise in the neighborhood, a new friend stopping by, a change in bedding arrangements, being able to see another cat near their home, a change in your schedule, etc. And, as you know, Siamese cats are intelligent and more prone to an emotional reaction.

I have a Siamese mix foster now who has had a major reaction to losing her home and guardian. She never got lipidosis because the shelter caught her weight loss early enough, but for months she wouldn't even try to eat herself, preferring to be syringe fed and even demanding it. She's been in my home about four months now and is just starting to eat normally.

I'm sure your vet will help with a good diagnosis.