Hi welcome to Health care magic forum.
Thanks for calling H.C.M.Forum.
It appears that your son of 4 years had stomach pain and vomiting
, no apetite. No response to medicines.
It appears to be lactose intolerance
, worms in the intestine, Or may the change of diet.
Try to change the menu to the previous menu, that is avoid giving the food recently added to the menu, like egg, meat, fish etc.
If no response consult a pediatric specialist. You may have to get stools examination for for worms and reducing substance, to find out any intolerance of lactose containing foods, like milk and milk products.
Wishing for a quick and complete recovery.