Definitely go to your GP if he seems to be struggling to poop. Although I've been told the 3 to 4 days can be normal for some babies ..
Tbh gripe water and sitting my baby upright for long helps her with constipation it always helps her
The advice is not to give Water between feeds. Speak to your gp constipation could be a few things for example he might just need some help with lactose or he may have an allergy, both mine have been constipated with cows milk protein allergy. It's horrible watching them struggle isn't it. Definitely worth gp visit.