It’s normal for babies to grunt etc, it’s called infant dyschezia. My little girl is breastfed and goes often, but she still grunts and acts uncomfortable, it’s just that they don’t know how to use the muscles yet. My HV also said that they are only constipated if their poop is hard x
Maybe try gripe water (can be used from 1month) to see if that helps with any discomfort? If it does then you may have an indication as to if it’s newborn behaviours or not x
I had this also, I switched from C&G to aptamil as my little one has pooped in 5 days after birth and my HV recommended I try a different formula and to try a pre made and low and behold he pooped after that. You can see he's uncomfortable try get a doctors appointment and get them to look, it's not nice to see your baby struggling and not being able to do anything to help and the fact your HV hasn't given any advice to help him poo is unreal. Good luck!!