Vomiting, feeling thirsty. History of constipation. Took laxatives. What is the remedy?

My son just turned 4 and gets severe constipation that causes vomiting he is being seen to treat this (just over a year now). He does take laxatives for this as well. He had a virus that caused diarrhea first on sunday once and then vomiting all night sunday and most of monday. Followed by more diarrhea since then. He has started vomiting again today and his diarrhea has a sickly sweet smell and is light yellow and or like water still ( diarrhea has been continuous since sun). He has not had any laxatives since sunday. He keeps telling me he is thirsty. I have tried just water, water with a tiny bit of juice, milk, he can't keep any of it down now. I am starting to get concerned.