Rapid weight loss but not sure why

Let me start off by saying this isn't about my cat, it is my best friends cat Gordon. My friend lives in KS and is asking for advice because the vet is stumped so far with Gordons loss of 4 pounds in two months. They had a full blood panel done along with thyroid and diabetes tests and everything has come back normal, his teeth and coat are still in good shape and he is not lethargic. He was on wet food but vomited daily on it so they now have him on science diet dry and he can have as much as he wants but is still not putting any weight on. She is worried as they put their other cat Margo down last year as she became very weak and faded quickly.

Gordon is 12 years old and only vomits once a week or so now with the food change and was 11 lbs and is now down to 7 lbs. Any advice is much appreciated and will be passed onto, I told her I'd come see if anyone can think of anything else to test for or foods to give him. I don't know if he had any scans done for cancers or the like.

Thanks to any responses, Gordon is a wonderful cat and we just want to see if something was looked over. I wonder if he isn't allergic to something in the wet food since he threw up a lot and is still vomiting occasionally on the dry as well. I suggest grain free or limited ingredient but I don't know what they want to do on that end.

Also he is indoor only and has had no problems other than the vomiting and now this weight loss.

Very sorry to hear about your friends cat!

It sound like there may be a possibility of IBD and you are right to suspect allergies. My cat also had severe vomiting and it was suspected he had IBD (the only test is a biopsy which is expensive and stressful). What really helped him was switching to a novel protein limited ingredient diet and I started with natures variety and now he is on stella and chewy's freeze dry raw. He can eat lamb, duck and turkey but gets really sick whenever he has beef. Another thing that has worked is vitality science supplements (http://www.vitalityscience.com/v/AllProducts/Cat-Diarrhea-Vomiting/feline-Diarrhea-Vomiting.asp) and shaving him to prevent hair build up. 

Here are some good articles on IBD (http://www.littlebigcat.com/health/inflammatory-bowel-disease/) and (http://healthypets.mercola.com/site...012/09/24/pets-gi-inflammation-treatment.aspx). 

Good luck!

Did he have x-rays or an ultrasound done? Not sure if there could be a mass or something that is not necessarily affecting the blood work. 

Thanks so much for the responses! I will get them to her and also ask if they did any x-rays or ultrasounds. Will be back with answer tomorrow as it's to late to contact her tonight.

X-ray, or better yet, an ultrasound is in order.... That much vomiting is not good.....
Too bad he is on dry food now, specially being 12. I bet this is either IBD or Allergies, best would be a clean, simple wet diet for him - it will be better for his health in the long run. Science diet is not good..... and something is going on, since he is still not putting weight.

Does she feed him ANYTHING besides the food?
Does he go outside? is he inside only?

They haven't done any imaging on him yet. He goes back tomorrow and she's going to ask about the IBD and some other stuff. I let her know that the science diet isn't a great food and it could be making it worse. she did say he hasn't lost any more weight which is good. I also relayed info on the sonogram helping weed out other serious problems.

I'll update when she let's me know how it goes. I hope he's okay, I used to cat sit for them when they went on vacation and he is one of my favorite cats ever.