Our son goes the other direction (too loose) but when I've needed (or done a cleansing system) I use psyllium powder mixed with 1/2 juice and half water. If you add a little oil along with it (olive) it will help even more. It is natural and non-habit forming. It also has some other benefits. A quick good search for more info.
Lots of water and backing off on constipating foods like dairy, bananas, cooked carrots. Adding more fiberous foods like whole grains, pop corn, prunes, spinach, unpeeled fruits and vegetables, etc. may help keep things more regular too.
Best wishes,