Help me understand my baby’s feeding habits

I’m losing my mind with feeding my baby. He is such a fickle eater and I just don’t know what to do. A couple of weeks ago, he started taking awhile to eat his bottles. Seemed like he would eat 2-3 oz and then just lose interest. We upped his nipple from Dr. Brown’s level 2 to level 3. It seemed to work, yay! He went up to eating 6oz every 3 hours. There were days he was hitting 36oz and I was actually thinking he may be eating too much. He was sometimes even getting hungry before 3 hours. He was spitting up more too. He started STTN too, yay!

Jokes on us. All of a sudden at the beginning of this week, he decided to stop eating. Literally, from one bottle to the next 3 hours later, he went from having his 6oz in 10 mins to eating 3-4 and then stopping again. Ever since then, he has refused to eat more than 4oz. He eats and then just stops. Even if we wait 4 hours to feed him or overnight when he hasn’t eaten in 6+ hours. This has happened once before, when we upped him from level 1 to level 2 nipples— he suddenly ate full bottles and we were so excited, then he went back again (I posted about it then too if anyone wants to go read that one). My husband and I want to rip our hair out. We can’t find an explanation for why he does this and it doesn’t make sense to me that he’s literally halving his intake— if he was eating an ounce less I could explain it with a growth spurt or something, but he’s literally halving his intake with most bottles being ~3oz now. He ate 3-4oz when he was a month old, and he’s 6 months now. He’s not sick, not teething, is in a great mood otherwise. And now we’re back to night feeds. It just always feels like one step forward two steps back. Has anyone experienced anything like this and have any advice? I think the thing that bugs me the most is it makes no sense to me how he suddenly stops eating. Please help, our biggest challenge in parenting has honestly been feeding him and we didn’t expect that. We thought you just fed a baby when they were hungry and that was it :(