Whats wrong with my cat?

I have a almost 2 year old cat and all hes been doing is just laying down.We have brought him to the vet 2 times. The vet doesnt know whats wrong with him. He hasnt been eating at all for the past few days. Hes usaually playfull and always wants to attack. 2 days ago my mom thought he was gonna throw up so she put him on the ground but the cat fell to his back and it looked like he was in pain. The first day he was acting weird he threw up twice.hasnt threw up since. I dont wanna put him down i love the cat! please help me if you know anything.