It's about 2 - 3 months after birth, and my baby is constipated and doesn't have many bowel movements.

As he gets older, the number of times each day your baby's bowels movement will decrease.
If he's not in a bad mood, hasn't lost his appetite, doesn't throw up a lot or have a bloated, painful looking stomach, you shouldn't be too concerned, even if he doesn't move his bowels for three or four days. If you are worried, try applying some baby oil
to a twisted up piece of paper or cotton swab and apply it inside his rectum. This can stimulate his bowel.If he hasn't had a bowel movement for close to a week, consult your doctor.