My son is vomiting. Always lying down.

My son started vomiting day before yesterday early morning as soon as he got up from bed. It was only water with yellowish in the last throwing up. Later, he was okay. Took only light food. But around 4pm when he drank water, he again vomitted. We consulted doctor in the evening and the doctor suspected it to be normal viral fever. He didn't had fever till then. Later in the night he had fever and we have Calpol 250 syrup. He didn't eat anything after that. Yesterday morning he drank water and then vomitted again. Later we started giving him water by spoon.. Only 2 spoon at a time. He had very little boiled potato at lunch and nothing else. He was continuously asking for more water. In the evening he once drank water by glass and after 10 mins he vomitted again. Since then till today 5 pm he didn't take any food apart from 2 biscuits. He was having water in sip.He is relieving little gas which has very bad odour. He is very weak today and unable to sit also. He is always lying down.