Mine had the same problem - we immediately switched to oatmeal and barley cereals, but it didn't fix it. A full jar of prunes always gets him better, but instead of that we started giving him juice every day. As long as he gets a cup of it he's fine - the days when we forget/or are busy he gets constipated. When he was younger our doc told us to try it diluted, but that didn't work so she told us to give it full strength and it worked like magic. Now we are able to dilute it and he's still doing OK now that he's older.
We also stopped giving him plain cereal - instead we mixed a little bit into fruits and veggies to thicken them up, and he still gets the cereal.
It will get better!!