2 weeks old not eating as much.

My son is now 2 weeks old and has had a big appetite since he was born. We've been struggling with constipation and switched formula to help (was breastfeeding but I couldn't produce no matter what I tried). The past couple days, he has went from eating 2-3 oz every 2 hours to 1-2 every 3-4. I try offering every 2 because he had a low birth weight and lost 8% when we left the hospital, but has picked up weight since. Ehh... I don't want him starving. He has plenty of wet dipers each day, so I know he's at least not dehydrating. Anyone else experience this?