Babys poo is green.

Once I had my lb I was expressing for the first week but had to stop due to falling really I'll with mastitus and inflamed womb, he was pooing after every feed but it contained mucas since I had to stop I changed to cow and gate formular 1 and he was really struggling with pooing every 4 days and it was dark green so my hv advised east digest at around 3weeks and it's no change the Dr had prescribed gaviscon but I don't know why as he's fine with wind it's just swelling of his tummy and pooing that's the problem.. if added a photo of his poo nappie if anyone can tell me there opinions? I know Tmi.. But I'm sure he's Intolerant to the milk..