Advice about possible CMPA

Our 7 week old was prescribed lactulose a few weeks ago as he’s constipated and has been since birth (we’ve changed his milk etc but still the same) we got told he’s got infant dyscezia however when we gave him the laxative his stomach made the most awful noises and he was in so much pain and unsettled I’ve not gave him it since. He coughs randomly like through the night or once he’s been burped and has started more often to arch his back when burping to lie back down, he whinges so much when we’re trying to burp him and it takes forever to get them up and his breath has started to smell abit funny and his dummy even though it’s sterilised about 3x a day. We went to the docs tonight as I spoke to the HV and she said it could possibly be reflux and to ask about baby gaviscon however, constipation comes often with baby gaviscon so the doctors have told us we should try the prescribed milk as he thinks he could possibly have CMPA (coughing, constipation and reflux being the symptoms) for 2 weeks and then put him back onto aptamil to see if the symptoms appear again and then we know it’s CMPA. I’m confused because I thought we would have been referred to a dietician first or something? He has no rashes and is never sick?? Anyone experienced this?