any advice or information for a first time mommy please!

so my baby is 16 weeks old. (3 months). I began breast feeding him at birth but I go in between with formula. I use Gentelease .

I wasn't producing enough breast milk. now about a week in a half im noticing he hasn't had a bowl movement. not any. getting worried , should I call his pediatrician or change his formula?if so to what kind? also he's been only on 4 oz as well since 2 months old. right now he should be taking about 6/8 oz but won't !..... idk what to do . his poop is green hard clay type. please any advice is wanted . it hurts to see him in pain.