I agree with Katy, pears work a charm for constipation, apricots or peach too but they can be a bit acidic for first foods.
I found around 6-7 months along with food I had to start introducing water to help hydration, just in a sippie, to start with they just play with it, but later down the track it helps a lot if LO likes water!
Apples and prunes are good as well
not to much prune though or LO will get a sore Tum .