I hope you figure this out. If the doctors can not find the cause, consider alternative medicine or energy medicine, acupuncture even. Search around and the right path will be shown to you.
He is only 21 years old. he has gotten two blood tests done and has given his doctors 2 different occasions of stool samples. they cant find anything. we are going to have to look into a colonscopy. He is only drinking water and gatorade. He isnt on an medications expect for a sinus medication called allegra and hes been on it for years. Its just weird that he feels completely fine during the day and then right before bed hes feeling sick to his stomach and has to go to the bathroom multiple times.
Start with getting a blood test done. If I were to guess, I would look at the liver. The liver when it is not working well will cause congestion in the head. Also the liver is the bodies filter and connected to the elimination system, so there is that connection. The vomiting and diarrhea may have been an initial infection of some sort that did not completely clear up.
Blood test, liver function, perhaps a colonoscopy. Get them checked out. In the meantime, examine what the man is putting into his body. What is he eating, drinking, exposing himself to. What medications is he on if any that might be causing this. You can go a long time feeling fine and then get side effects from medications, or anything else for that matter. An age would have been nice. Good luck.