L.B.
We did the prune juice too but diluted a little bit (we didn't want him to blow out...lol). When he got a little older he started refusing those bottles b/c they were "different" so we started calling his bottles Vanilla and Chocolate...no problems now! And imagine! He prefers Chocolate to Vanilla now (he he he).
Another thing to try is little tummies laxative drops. Haven't had to use them too much but when I've really needed them they have worked wonders!