Hi
Just wanted to add my story. Will try to keep it brief.
My youngest son (now 2.7 months), did not poo until a week after he was born. He was breastfed for about 8 weeks and then bottles slowly introduced. He also struggled to go for a poo - I had to do stomach massage, leg exercises, anthing I could do to try and help him go. He was always uncomfortable, writhing around at night, screaming in pain, would never finish a feed.
When he was a very young baby we took him to a cranial osteopath as he was never settled, he did some work on his head, but always told me he was constipated! The osteopath would do some work on his tummy and this would always lead to a poo or 2 or 3 later in the day.
Basically, the constipation continued to get worse. I kept saying 'he will do better when he crawls/walks etc', but unfortunately he did not move anywhere until he was 1. When he became a bum-shuffler Did not walk until he was 18 months and this made no difference to the constipation.
He got so bad that when he was straining to poo he would sweat buckets and vomit small amounts He would soil a lot and then eventually (maybe once a week) manage a proper poo! This always involved a lot of screaming. We were given lactulose first, but this did not touch it. Eventually, we were referred to a consultant who put him on movicol. This worked like a miracle, high dose to start off with to clear him out, slowly reduced it and has remained on a sachet a day ever since. He now has regular bowel movements and only soils every now and again. (Just starting to potty train, but that is a whole other story!)
My son also had a bowel biopsy to rule out Hirschrungs Disease last summer, this was done under a GA at Adenbrookes Hosptial. Luckily, he does not have HD - just chronic constipation But, as you say, at least we know what we are dealing with and treating it correctly.
I hope another story helps. I def think you should go back to your GP and try and get a referral to a Paed Cons, so you can explain your son's medical history in detail. As a starting point for the constipation problems Movicol could be perscribed by your GP, it really does work well. Hope you manange to get something sorted for him.