It seems that your son is having acute gastro-enteritis
giving this problem.
As he feels thirsty, there is a chance of having dehydration
Consult your doctor and start treatment as soon as possible.
Meanwhile give him plenty of water, apple and Dadam juices.
Give him ORS
liquid and electrolytes.
Give him light diet like curd, rice and buttermilk.
Ok and take care.