No poop for 4 days

Hi, my 6 week old hasn't pooped in 4 days - his usual is 2-3 days, but, I'm thinking he might have a reason for this? It's currently 4am so can't get in touch with Dr's just yet, just looking for ideas.

He's formula fed, has had wet nappies, not sodden but wet, limited intake of milk cause it's getting to 4 days.

He also has baby acne but I'm thinking now it might be eczema or a rash that could be CMA?