Talk to Pediatrician on 4 Months Baby Vomitting After Having Formula Milk & Weakness

Hi,

My son (he ll be 0f 4 months age on 18th of May 2020) as a practice vomitt after drinking Lactogen 1 formula milk (from his birth he rarely breastfeed with his mother). Yesterday and today vomitting was on higher side than usual, and as of today we are worried for his health as he got v weak in a day or two. today he had little fever as well,his mother gave him panadol, and his body temperature is fine now. Today, his monther and i decided to change his formula milk and bought BF-1. kindly advise us (1) should we start feeding baby on BF-1 (replacement with Lactogen 1) on by ourselves or should we take him to a doctor and only after doctor approval can we change his formula milk? (2) kindly advise as well what should we do for baby weakness after this heavy vomitting so that he can recover early as possible? from his face, his body he looks weak..normally he is a v active child.

jazak Allah !