Feeding advice 4week old

My lb is four weeks old and since going onto formula at 12 days due to weight gain issues he has got progressively worse at taking feeds and is showing digestive discomfort through out his sleep which is waking him up. During feed he is crying screaming throwing himself around crunching up and pushing himself straight and cough/spluttering. He settles on me in a upright position on my chest but as soon as I lay him down he starts grunting, squirming, going red in the face, pushing out farts, moaning and eventually wakes himself up. This is at its worst on night and very early morning. Still unsettled in the day but not as bad. He is filling nappies regularly. HV says it’s just normal but like I said it’s getting worse. Considering trying the Aptamil anti reflux milk, what are people’s thoughts? He is currently on Aptamil first.