Weight Problem

  My cat Surt male and neutered. He is about 5 years old now he was at an ok kinda weight as a kitten and then we moved a total of 5 times with him. We are back in a familiar house and he seems very happy and energetic but he is just skin and  bones. I have 8 cats total they all get along and he is 1 of 2 siamese mix of the house but the other siamese is not even 2 yet but weighs more than Surt. They have all been wormed but Surt is the only one with the runs and kinda refuses to use the catboxes anymore even though they are always clean. I haven't changed cat litter type or boxes and its rare that he poops under the table but I'm really worried about him. All the cats are strictly indoor they do not go outside at all. They have been vaccinated and are well taken care of. Vets around here though treat me like I'm stupid and won't listen to my details all they want to do is run test so that those straight out of college can gain more experience. My mother has been a dog and cat breeder even before I was born and Ive learned alot in the time that Ive been alive. They keep blaming it on worms but I know its not that. Im low on cash and would like some suggestions. I really dont want to put this cutie to sleep just yet he still has a lot of life behind him if I can just figure out how to fatten him up.All the other cats are a really good weight and dont have any problems of their own.

What you don't tell us is if he's eating the same amount as always.  If a cat ( or any animal) is eating the same amount as always, and losing weight, then there is a problem.  Even though he doesn't go outdoors, it's still possible that he has worms.  Tapeworms, for instance, come from fleas.  Fleas can be brought in on pant legs from just about anywhere.  Other infestations can come from mosquitoes.  Have you taken in his stool for them to at least do a fecal float? 

It could be his thyroid.  It could be one of many things.  Have you at minimum had his blood values checked when you  had him seen last?  At minimum I would say that needs to be done.  I don't see any reason to think he needs to be put to sleep just because he is losing weight.  There very well could be a solution for it right around the corner.  But before a solution is reached, you need to find the cause, and tests ARE normally needed to figure that out.