For the past 3 months, my cat has been losing weight. He looks much bonier then normal. He was not eating very much for several months. He is a 12 year old male, indoor cat. I got a new kitten and had them interact for several weeks, around the time when this began, and he did not like the new kitten, he was very mad and isolated, I gave the kitten away do to what I thought may be causing him stress ( I don't know if the kitten was related to him stressing and not eating.), but a month later he still has not recovered I took him to the vet, they ran blood tests and it came back clean. They gave me antibiotics to give him, as well as vitamin supplements, and pills to enhance his appetite. He has been given these as prescribed. Now the past few days, He is eating, and 30 minutes later he throws up. Also his weewee is sticking out, and it is very odd.
It is a worrisome situation.. What could be the problem?? Any information would be grately appreciated, Thanks!

Did they check his thryoid by chance? He could be hyperthyroid.

Older cats can develop kidney failure or liver failure. Male cats can also develop urinary tract infections. Some older cats need a specific food to help in this area. Like the other person mentioned, he could be developing a thyroid problem, or even diabetes. Diabetes can present with sudden severe weight loss, but also a LOT of water drinking and urinating. He needs to be checked by a different vet and urgently so - with blood tests and urine tests. Also, get the vet to check his teeth. Cats tends to develop abscesses in their teeth and because of the pain will refuse to eat, or alternatively gobble down whole kibble and then puke it back up again because they went without food for so long and then overwhelmed their system. Eventually the teeth tend to fall out, but it is reasonable to check his teeth to see what's happening in there.

I hope your next vet can find out what the problem is.