A good friend sent me a book, which was a true story, about a family who took their two small children to live in France for a year. The author spent a lot of time talking about the youngest child and how he cried morning, noon, and night. There was no suggestion a doctor was asked for advice and I thought about what I would have advised the parents to do. Obviously, something was hurting the child. The author noted that the child suddenly stopped crying when he was about two and a half.
My advice to the parents would first have been to stop all milk products. Many little children cannot tolerate cow's milk, but develop a tolerance often, as they grow older.That would explain the time when the little boy stopped crying. If eliminating milk products for two to three weeks didn't help, then I would have suggested slowly eliminating allergenic foods for two weeks at a time. The other way this can be done is to stop all of the possible offending foods at the same time and then slowly introduce each one back again.
Ideally, a pediatric allergist would be consulted, but if the parents were not willing or couldn't afford to do this because of lack of insurance, eliminating milk and certain foods makes a lot of sense. The foods that cause the most problems with children are: citrus fruits, eggs, wheat, fish, nuts, and chocolate. I have seen amazing results when allergenic foods are eliminated and much happier children..