Siamese Cats and bad breath?

Siamese are notorious for bad teeth. Get him in to the vet for a dental exam, I'm betting he needs his teeth cleaned or worse.

Some cats do have bad breath. My friend has a Siamese/Colorpoint and her breath is not bad. I'd suggest taking him to the vets to make sure he doesn't have a gum or tooth infection, which could be serious. Then get him a dental cleaning and continue to brush his teeth consistently with cat tooth paste to what you think he needs (weekly or monthly).

Cats build up tartar on their teeth. This causes bad breath. Also, If left untreated, they will eventually loose their teeth. The vet will remove the tartar but the cat has to be sedated. This can get costly. Try taking a cotton round and dip it in some tartar control mouth rinse such as Listerine and swipe the teeth and gums. Do this on a regular basis, daily.
Sometimes changing the diet helps. There may be food supplements available for bad breath control. Check with your local pet shop or your vet.

This may be caused by infected gums or teeth. Get your cat's teeth cleaned and train him so that you can brush his teeth yourself.