Talk to Pediatrician on Vomit Meals

My son 16, months old , already underweight but very active... He sometimes vomits at the end of his , wht shuld I do, I jus cry, after feeding him for one hour, one vomit ejects all, does big vomit let some nutrition or not????? I cried a lot, is there a method to stop vomit, I feed him very slowly with proper interval as well... Now, with empty stomach , he wants to sleep... ;-(