I am starting to feel like maybe my BFing is doing him more harm than good.

His poo remains very green and completely watery and frothy alot of the time (I caught it on exit today!) but the big change in teh last few days is that while before he didn't seem to be suffering digestive problems too often (maybe once a day for half hour or so) now he is really suffering after almost every feed about 45 mins later with what looks from his body and face and the screams like really bad pain and wet hiccuping, hiccuping, gassiness. He gets totally hysterical and the only thing that calms him instantly is more boob, though I am trying not to give in and just to sit out the screams as I am conscious that would probably just adding to the problem.
Feeding from one boob only the poo is still green but definitely better consistency though he is still gassy.
I have seen improvement in poo consistency and colour the couple of times I have been able to pump pre-feed but it's not always possible for me to pump for practical reasons, in fact, it is downright difficult.
My question now is, will I always have this problem when I BF and always be giving him too much foremilk despite my best efforts and need to be scrupulous re which boob he eats from and pump pre-feed or is this just a phase? Another question is, should I offer the second boob after he has gone as long as he wants on the same boob or just stick to one?
Feeds from one boob are still mega long with him, he moves to the fluttering stage at about 30 mins. I am still having problems knowing when he is finished eating, often he screams when I take him off even after he has been on 25 mins but then stops screaming very quickly within seconds making me think he was full but I then have paranoia that I haven't fed him enough and offer the next feed too early probably and make things worse probably....
The thing is, I don't have boobs that spray or leak so I don't know for sure that I have an oversupply but it is my instinct that I do and I have alot of the other symptoms - frequent blocked ducts, strong almost painful letdown - gassy unhappy baby with green poo!! Will things ever right themselves?