What's going on with my bloated and tired kitten?

My 14 week old kitten is acting ill and I took him to vet twice and they said there's nothing wrong with him. He was diagnosed with roundworms for having bloated belly, diarrhea, gas and lethargy. Two weeks later I turned in his stool sample and it came in negative for worms but his on and off symptoms is still there. He sleeps all the time and unleashes really bad gas all day. He was well-fed with holistic canned and dry food. His belly is really bloated and it felt like air is trapped in it. His temperature is 103.1 f which is a sign of fever. He looks really ill and it just doesn't make sense at all. Sometimes he would act really sick and won't move, but the next minute he would play around and pounce. I kept the house clean and have his vaccination up to date. He's like that for a month. What is it??