The cause for vomiting
and diarrhea needs to be known at first to start with any treatment.
Till then give him homeopathic medicine Arsenicum alb 200, 4 pills thrice a day for 3 days and Ipecac 30, 4 pills twice a day and report thereafter for further treatment.
Give him light diet
like khichdi or dalia, give her
ORS 3-4 times a day. And revert back for further treatment.