Formula question

My LO is just 6 weeks 2 days old; he was feeding 5oz formula on average per feed until last week Sunday and all of a sudden we have issues with feeding, he will have the first 3oz easily no issues and fights the bottle after that. It takes forever for him to finish the 4oz Max now.

I try burping him and changing him half way through

He was such a good feeder, and now it’s a bit of a fight between him and the bottle every feed.

I am just asking; is anyone else experiencing this? I thought as they get older they meant to take more milk, he has reduced his intake!

I am a little worried about how much he is getting per day; I might also add I changed the teat of the bottle to a size up, and he cried a lot and didn’t drink so we went back to the original teats.

Any advice is appreciated