My Son, 25 Yrs Old, Has Been A Vegetarian For

my son, 25 yrs old, has been a vegetarian for six months now. He went to Texas to visit family for a week and half and ate alot of meat and came back to cali feeling ill, constipated, etc. I gave him some milk of magnesia and was cleaned out and stopped going two nights ago. He started going again yesterday all day and again this morning. When will it stop? And could he be drinking too much water?