Try rubbing his belly and pumping his legs gently when you are aware that he is trying to have a bowel movement. If it doesn't get any better by the time he goes for his next check up, talk to the doctor about what you can do to help him. The doctor might know of something you can give him to loosen his stools.

My little man had the same problem, my pediatrician said to take 2 ounces of unflavored pedialite & a teaspoon of dark karo syrup, mix it up and give a little to the baby.

He hasn't had a hard poopie since!

My step son had the same problem but his carried on for a long time. We were told to try giving him a small amount of flax seed mixed in with his food and it worked like a charm so give that a try.
P.S it tastes like crap so make sure it's in something that he enjoys