Many cases, formula can cause constipation in babies. We switched to Aptamil formula as suggested by our paediatrician and it was perfectly fine for him. It has prebiotics too.

Just lookout for 2 more days Anon,if he doesnt poop,talk to your paed. Its okay for this small babies to poop once in 7 days. Meanwhile,apply heeng paste on his navel twice daily. Make him do cycling motion exercise,bend his knees toward tummy for few seconds and repeat 5-10 times,burp him after every feed. This all will help him relieve gas which might loosen the bowels. You can take ajwain water,dates,fennel to ease out his bowels. Plz check with paed once,he might suggest a digestive enzyme.