Hello, I Have A Son Who's 15 Years Old. He's

Hello, I have a son who's 15 years old. He's been experiencing gastrointestinal problems recently for the past 6 months. Usually, the problem is that he feels nausea and diarrhea specifically in the morning. This causes him to either throw up, or intentionally make himself throw up in order to relieve the nausea. This is usually accompanied with diarrhea and sort of painful bowel movements (not horrible pain though). I don't know why this issue peaks in the morning and why it has been going on for so long. However strangely enough, by the afternoon, he feels completely normal and fine. He eats normally and in all honesty, he watches what he eats very well and really makes an effort to eat healthy foods and skip out on greasy and sugary foods. He has been tested for parasites and a few other things but nothing has shown up yet. Hope someone can give us a second opinion.