My guess is that the baby is not being breastfed, as nursing infants rarely have constipation. I wonder what else is going on? What kind of foods is this baby eating? Medications? Drinks? activities?
Some ideas that have helped mothers in the past.
Massage the abdomen of the infant clockwise, moving legs around can help, too.
baby should be drinking a lot of water.
An enema using one of those blue bulb ear cleaners that they give at the hospital might assist. Use a liquid acidophilus/purified water mix and fill the bulb with one -two squeezes. (not all the way full)
Insert bulb into anus and squirt once, remove quickly. It helps to rub baby's belly. If poop doesn't come out in 15-30 minutes, repeat.
I've done this with my own infant (when she was ill) and then on another 4 month old that ate too much food for her age.
The results were that they both pooped, and they were smelly, like the excrement was putrid and inside for too long. The poop was forceful, and the mother had results within 15 minutes, thank goodness the diaper was on the babe.
Hope this information helps, but you must use your own judgement when applying.