He is peeing fine but not pooping

That does not sound good. Call the pediatrician

How long has it been? Are you nursing or bottle feeding? If you're using formula, are you using one that's high in iron?

When he did poop was it greenish, constipated looking?

The general consensus seems to be once a day at least. Though some can poop up to 10 times a day.

Something that the doctor told me to do, and it worked for my daughter when constipated was getting Vaseline and taking a rectal temperature.

Some babies just need some extra stimulation on the rectal muscles.