well search for a really good vet in your city.
did you notice any lump in his stomach? Coz such scenario happens of there is any blockage in stomach. did you change the diet? try to give him few spoons of yogurt and see. yogurt eliminates the sick belly normally.
But would strongly advise to find a good vet and have his ultrasound done. I guess there can be some tumour/blockage in stomach.