What Is The Treatment For Diarrhea And Headache?

my partner for many yrs has suffered from diarehha, he was off work for 3 months as he could not keep any food down and lost lots of weight, he introduced food slowly, and thought it was onions and tomatoes, but he still suffers from this but not as bad, im still concerned, he is always thirsty, sleeps alot, get regular headaches, could this be diatbetes ???