One thing you can try is pear juice... If you go with the fibre route, I would give you LO plain water with it. Pear juice works just like prune j. but is more gentle on the tommy. You shouldn't have to do both unless your lo is really bad.
My Ds was constipated really badly from 4 months until around 2.5 years (if I didn't constantly stay on top of the situation) I found fibre and juice was a really strong combo. If you want more fibre you can also increase fresh fruit.