Doctor suggests 5Ps, plum, pear, peach, peas, prunes. Apple juice also helps.
You can also go to see the doc and they will give a safe medication to let the baby poop to clear their system within several days. Then you should change his diet to give him more water and fiber food on a regular basis.
I used Flax oil on mine for a while. It helps. You can put his little butt into a warm (little bit on the hot side) water, that will let him poop too.
Read the below link from Dr. Sears. http://www.askdrsears.com/?q=topics/childhood-illnesses/constipation