What's happening with my son's eating routine!?

My son takes half breastmilk and half formula, sometimes just formula and sometimes just breastmilk it all depends on how much milk I've managed to pump.

He's 5 weeks tomorrow and for just over a week he's been on 4oz every 3 hours then it went down to 2 hours.

We ended up changing the formula because it didn't agree with him and since the new one he's been way better. He's hardly sick now, he seems way more content afterwards.

However now he will take 2oz and refuses the rest but like 20 minutes to an hour later he will want the other 2oz.

Last night he was having his 2oz refusing the rest but then woke me 20 minutes later for the other 2.

When he was taking the full 4oz he was going 5 hours in the night.