He was blood tested. He's on pills for hyperthyroidism but it doesn't seem to make a difference. They also checked for diabetes (I'm diabetic myself so it's always my first suggestion! ) but his glucose is fine. He's so fussy with what he'll eat..only a tablespoon at a time..but he's been this way for over a year so maybe he's just a skinny minny. The vet checks him regularly. My other two are tubbies so maybe it's just the contast.
He seems happy and a chatty siamese. Thanks for the input!