Hi Emma
When you are breastfeeding it is common for babies not to poop for anything up to 5 days!!
My eldest Jacob once went the whole 5 days without going - he wasn't in pain, he was still wetting his nappy - he just didn't need to go!
The fantastic thing about breastmilk is that because it's tailored to your babies exact needs - there is sometimes just no waste for a few days! It's that good!
At least, that's what my midwife told me anyway!
My 2nd son Benji passed his first meconium poo - then didn't go for 3 days after he was born.. that was a bit more worrying but he went eventually and was non the worse for it
He then just went once a day at the same time
- made for interesting dinner conversation as it was 8 o'clock in the evening when he started erupting and would often take 10 to 15 minutes to finish !
That said however - I agree with Julz - it might be the formula bunging him up a little. If you think it is, he will be noticably uncomfortable and I would recommend a little olive oil on his tummy and rubbing clockwise to help get things moving if he seems in pain. Talk to your HV or MW about the easy digest formulas available.
I used to drink a pint of orange juice when I was breastfeeding if the boys were actually constipated - worked a treat too 
Hope you and he are doing well 
mG & boys x