What could be the cause of diarrhea in a one-year-old child?

Hi doctor,

My son is 1-year-old with a weight of 15.43 lbs. When he was 7 months old, we started to give him solid foods and I gave him cow's milk for three to four days. After that, diarrhea got started and he passed stool twenty to twenty-five times in a day. So the doctor advised us to admit him in the hospital. After getting discharged, he passed stool three to four times a day. He does not had any weight gain. So I asked the doctor about it. He told me that he seems to be fine and he will gain weight after a month. There was no improvement in his weight.

I consulted another doctor and he told me that my son has lactose intolerance and told us to stop giving milk to him. So we stopped it for 15 to 20 days. It did not help him. The doctor advised me to avoid taking dairy products as the baby taking mother feed can have it. So I stopped taking dairy products and stopped giving mother feed for twenty days and it does not help my son. We did many tests like TTG, IGA, IGE, DGP, CBC, stool routine, urine culture, magnesium, calcium, zinc, phosphorous, and fecal elastase for my son. But everything was normal. Please suggest.