Last night he settled easily and when he woke up after an hour he was screaming and unsettled
^This sounds like OT, its pretty classic that those waking shortly after going to bed are related to OT at BT. Could also be gas, which is typically causing that pain around 1hr after a feed.
I thought maybe I should try feeding him just before sleep (an extra feed) which resulted in 20 minutes of crying and then I gave him some infant's formula, the safe settling forula for windy babies and he settled almost immediately and is now asleep (no idea why- it didn't work that quickly).
Was he perhaps looking for an easier feed? Bottle flows and requires less work than breast (opposite for my DS because of my crazy letdown, but generally true). Has your period come back? That can make your milk taste a little different and LO may well feed less through those days until they get used to the variation.
he was 75% percentile, now is around 30/40
Has this drop been sudden (over the last month, say) or gradual from birth?
sometimes he is pulling up his legs etc and I wonder if it's because he really needs that full three and a half hours to four between feeds?
This sounds like gas - it can help to push his knees gently up into his tummy and give him a little tummy massage rather than feeding which perpetuates that cycle. Of course, feed if he's hungry but I found once I helped DS get the gas out, he was much happier without those extra feeds.
I am expressing when I can because I think I have enough milk but he seems to get distracted feeding v easily and I suspect maybe he has always had a slight tongue tie which means his latch has always been ok but never great, so that combined with being interested in the world means he just doesn't take as much and so the formula and EBM in the bottle seems to help.
Sounds like a pretty solid theory to me.
Today we've decided to only feed him in one sitting, not to wait and top him up.
I think given you're topping up for volume reasons, that's a good idea. If you were topping up to keep the gap between feeds smaller if his routine was in one of those in between stages, it would be more appropriate to do as you've been doing.
In the morning when he wakes I give him 1-2 teaspoons of rice cereal with milk because he's so interested in food, but that's really just a practicing thing- he doesn't get much from that.
Is this before or after his milk? Do you think it could be causing some digestive discomfort? It can be a bit much when LO's system is so young, even just the feeling of something more solid traveling through the gut can have a LO squirming and crying - not necessarily painful, just different.
I think in summary, if you can get the feeds consolidated and maybe spaced out a bit more by helping him get rid of the gas when he's needing that, increasing your A times should then be a bit easier and hopefully things will fall into place for you.
Keep in mind LO's A times keep increasing, so by 6 months, they're up around 2:45-3hr, so just keep increasing by ~10-15min every week or two as tolerated and that will help avoid getting into another mess around 5 months