Bottle feeding issues

I have a preemie who is mostly breastfed, but I have to give him a couple bottles of calorie fortified breast milk a day. Sometimes he seems to have strange problems taking the bottle and I don’t know what’s wrong or how to fix it. He’ll suck like normal and then he’ll start scrunching up, almost like he needs to poop but he doesn’t. I have to take the bottle away when he does this because he holds his breath and starts to choke. He’ll do this multiple times during the feeding...like every 2-3 minutes.l and it makes the feeding take twice as long. I don’t know why he does it. He doesn’t do it every time, so I don’t think it’s a problem with the bottle or nipple flow. I’ve tried both a preemie nipple and a level 1. It makes me dread every time I have to feed him by bottle. He also leaks quite a bit of milk out of the corners of his mouth so he has a lot of milk loss when he eats. I always warm up a couple extra mls so he’s getting everything he should. That happens every bottle. Anyone have any suggestions?