I haven't had this problem with my boys, but I did read up on it before I answered this response. Firstly, all the websites said that you need to increase fiber in his diet. For kids, this is most easily done by either adding bran cereal to his diet, and/or increasing the amount of fruits and vegetables you give him to AT LEAST five servings a day. THese fruits in particular seem to be the most helpful: apples**, guava, pears, grapes, papaya, apricots, kiwi, and orange juice..and make sure he drinks a lot of water. Hope it helps. I haven't tried any of these, but I know the increasing fruit one DOES make my kids have softer stools..so try the bran cereal/fruit,water thing first.
Another home remedy I found was to add 1 TAblespoon of corn syrup to water, or to try adding a little sugar or honey to a glass of milk and drink twice a day. Good luck!!