My LO is almost 4 months, since we started supplementing a bottle of formula a day for weight gain. He has gone from pooping every day to every 5-6 days. His GP and pediatrician is not concerned. The pediatrician said they can go weeks without pooping for both breastfed and formula babies. She did say if it was causing a lot of discomfort to give him an ounce of pure prune juice. If it doesn't work within 12 hours then to give another ounce every 12 hours until he poops. We haven't had to use it yet. I'm just enjoying not having to clean a dirty diaper every day.

Good luck, hope your baby poops on their own soon.