Vomiting at night diarrhea during day

He is almost a  3 year old boy who started vomiting only at night (up at 7 times a night) followed by diarrhea during the day( varies amount, but alot).  He complains of stomach aches and has very little appetite. No fever throughout this.  He has been doing this for the past 15 days.  We do have a 1 or 2 days that are good but then it starts again.  He is on a very bland diet.  He was just tested with 4 vials for rotovirus, parasites, etc but it was all negative.  Could the tests be wrong?  I thougth for sure it was a parasite but it come back negative. He does eat a lot of outdoor things.  What should I do to help him?