Have you tried giving juice. There are some in the baby isle at the grocery store and you don't need to water them down. Consult your pedi and maybe try a few ounces of pear/prune juice for a couple days. We've been using it since DS was about a month (1-2 oz/day) upon our pedi recommendation. It seems to work in a day or two.
Prunes, pears, peaches, Yogurt(bc of probiotics) and we've also used baby probiotics that we put in water and give to our 8 month old if he hasn't gone in a few days. Good luck!
Re: Constipated