Hi E.,
You should definitely see changes in his poop with the start of solids. However, there are things you can do to make sure he stays regular and doesn't get constipated.
Rice cereal is one of the simplest forms of food to introduce first (easy on the digestive system), but tends to stop a baby up. So, when my son was ready to move on to a new food introduction, the first thing I did was move him to a higher fiber, but still simple, cereal like oatmeal. In fact, once he was "done" with rice cereal, we fed him primarily oatmeal and barley cereals as his cereal. Then, when we introduced veggies (first) and fruits (second)---highly recommended in that order b/c they don't get the taste of "sweets" first and then refuse veggie baby food---we rarely had any problems.
I remember only twice having to deal with his being constipated, at which point we mixed 1 oz of prune juice with 3 oz of breastmilk/ formula to make his then-4 oz bottle.
I hope this helps!
D.
Oh, and I've read that oranges shouldn't be introduced until 1 year old due to the acid in the citrus being hard on their sensitive stomachs. I'd ask your pedi if he/she agrees before feeding him oranges...