I'm old fashioned but I have raised three of my own and have 8 grandchildren. Is your baby drinking any juice at all? Sometimes a little pediatric strength juice helps - don't give adult juices. I'd probably even dilute the pediatric juice with some filtered or bottled water (no additives) and try about 2 ounces. I know most Pediatric doctors today don't like to start babies on anything but formula or breastmilk until they are older. Another old fashioned remedy is a tsp or so of dark Karo syrup in their formula. Hope it helps.
First, I think, you should find out, what causes constipation? Babie's stool is hard, is he getting enough fluid?
Brestfed babies are rarely constipated. If constipation is only from time to time, there's also not likely, that there would be some undrlying disease.