How Long Should You Feed A Cat Kitten Food?

(please help)

My kitten is now 5 months old and he has been really sick for the past 2 weeks. I took him to my vet yesterday because of his poor appetite, vomiting and diarrhea. They ran a couple of tests on him and said everything was okay but I am worried that it’s something more serious than they think. He had a stomach X-ray done at the same time as well which came back negative for anything major (like an abscess or blockage). He still won’t eat, he seems lethargic all day long and keeps getting up from his basket to go outside every 3 minutes due to the pain in his belly area where he gets up too many times. Any suggestions?