Welcome to HCM.
Pain in abdomen,vomiting indicates some intestinal infection.
Give him anti emetic like ondencetron mouth dissolving tablet.continue ors formula,give lemon water with salt and sugar,buttermilk.
Start proper antibiotic like ofloxacin,norfloxacin
under guidance of your doctor.Zinc preparation will be helpful.Take care about his diet.Give him light,liquid diet
.Keep in touch with your doctor.