I have changed my diet recently to cut out dairy as he has a CMPI but not eating anything new. The weird thing is that he feeds fine during the night. And he normally latches fine at the beginning and gets milk as I have a strong letdown on the one side, so I don't know if SNS would help, as he gets plenty of milk if he sucks even a little-half the time it pours into his mouth without sucking anyways, but he doesn't seem to be choking; it's usually part way through the feeding that he screams and fusses, even if he is still obviously swallowing milk. It's like he gets bored and frustrated.
Hi Blackrain!
[EDIT: Whoops I think I got my wires crossed when reading posts because I originally thought you were supplementing with formula and gave advice relating to formula and allergy, very sorry!]
This page might be helpful regarding his CMPI:
https://kellymom.com/health/baby-health/food-sensitivity/
One thing that might be affecting him is if dairy is still slipping into your diet somewhere. It's in absolutely everything! I really feel for you because it's hard to avoid. Perhaps an idea would be to keep a food diary, and you can go back and see what you've had when he's doing this and whether there is any hidden dairy.
Alternatively, does he show any symptoms of reflux? He could be having acid come up midway through a feed, and that's why he's pulling off, especially if he's arching his back when he does it.
To be honest though, you might never find a reason as to why he's starting doing this. I often found myself going through these periods of fussiness with my first baby, and would drive myself mad trying to pick apart the reasons she could be having problems and what I could do about it. In the end, those periods always passed and she'd go back to normal. It's so tough to deal with though.
I hope it's ok I just jumped in and offered advice! I don't feel like I have the time to offer to be a champion right now, but I feel like giving advice here could be helpful, if that's ok.