Baby reflux

I have a 4month old and I'm not sure but I think he might be suffering with reflux. He is formula fed (Aptamil) and I'm thinking of changing his milk to anti-reflux Aptamil as he seems to spit up milk while feeding and quite often after he has eaten up to two hours after he was eating. I know babies tend to spit up milk sometimes but about 80% of time he eats he gets really anxious pretty much since he was born and cries a lot because of it. He starts to drink and calms down for a bit,then starts crying but wants more milk and after a while he kind of calms down but gets really annoyed and irritated easily while eating. He spits out quite a lot while eating and it normally has little bits of formula on his bib or chin. If I do change his formula do i do it gradually or can I change it completely straight away? Thank you for any advice in advance xxx