See your pediatrician before doing anything yourself. Babies bowel habits can be varied and a doctor needs to determine if the baby is really constipated or if this is a normal pattern. I would not do anything until checking with a doctor first. Just keep up with the bowel pattern to be able to give the doctor an accurate report.
Babies can go up to a week without pooping and it still be normal. It doesn't necessarily mean they are constipated. If he really is, then try doing a little bicycle exercise with him for about 5 minutes or so a couple times a day. If he doesn't poop after a week, call your pedi.