Is something in my 9-month-old's diet causing him to pass very hard stools?

Yes: A lot of 9 month old babies begin to rely more on food and less on liquid for their daily nutritional needs. Very often, a baby gets a bit constipated at 9 months due to a decrease in their fluid intake. Adding some extra formula, breast milk, water, or even water mixed with a small amount of prune juice or white grape juice can often solve this problem. If it persists, consult your doctor.

Yes: More like a "lack" of something: water and fiber. Fruits such as plums and bananas, and lots of extra water (not juice!) will help.

Sometimes: I find rice cereal, bananas, apple sauce all have a constipating effect depending on the amount given.On the other hand, barley cereal, apple juice, prune juice, peaches,pears all have a loosening effect.