Hi
The cause of hard stool may be due to all three causes.
You have not mentioned what type of solid foods you have introduced.
Please check his diet and ensure that he get more
dietary fiber rich foods (green leafy vegetables, whole and shredded wheat, grains, unpeeled cooked vegetables etc) and less low-fiber foods (like banana, milk and dairy products, meat, refined cereals, white bread etc).
Ensure that he drinks sufficient fluid and water (2-3 small glasses).
Dehydration is the cause behind
constipation in most cases.
Maintain a good bowel habit. Let your child a lot of time in toilet if he is not passing stool. Retained stool get dried and hardened. It is more difficult and painful to pass a hard stool. The child try to retain that, which actually cause more constipation. Thus the vicious cycle goes on.
Check the dilution of the formula. Prepare the formula as recommended. Don't make concentrate formula solution. Some composition of formula may cause constipation in some babies. The formula should be changed in such cases.
Medicines like
lactulose, suppository are available to relieve acute cases of constipation. But don't use these on regular basis.
Constipation is very common at this age. Please follow the recommendations as I mentioned. If still the constipation continues, visit your
pediatrician and get your child be examined and treated accordingly.