Welcome to HCM.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided.
It needs to be evaluated as you have mentioned he had been like this since few months. Stool routine and culture should be done. Lactose-hydrogen breath test
, Lactose tolerance test, Stool pH test can be used to rule out lactose intolerance
I advice you to give him plenty of fluids as vomiting
is not the issue, he should be properly hydrated. I advice you to see child specialist for examination and exact diagnosis. Presently i don't think it is wise to speak about the medication without examination as he is only 14 month old. I sincerely hope that helps.
Hope I have provided the information you needed.
Wish you good health. Take care.