S.A.
My youngest son who is now two has the same problem. when it first started (around 15 months of age), I thought that a little pat on the behind would make him stop. That was the worst thing that I could have done. It taught him that hitting was acceptable if you were angry. The best thing that I have found is to put him in his room alone or just completely ignore him. Because I did not start taking care of the problem right away his hitting had progressed. I hope that with consistency he will eventually stop. Hang in there!