Hi S.,
You are asking the perfect question, "What is causing him to do this?". So often, we see a behavior we don't like and we stop the behavior, through punishments and time-outs. Those are ineffective and, ultimately, do nothing to identify what lies at the root of the problem.
Imagine yourself being 19 months old. The world is much too big for you. You are completely dependent on adults and, most importantly, you feel as though you have no power. You can't always communicate what it is you need or want and sometimes, the only control you have is to throw a tantrum or hit someone.
Hitting is not a bad thing. It allows our children a physical release of their frustrations. What is a bad thing is "ouch-ing" Mom or Dad. Next time your son tries hitting (or threatens to hit) acknowledge his need for it and find a way for him to hit something without hurting anyone. He can hit a pillow, he can hit the floor, you get the idea. Trying to prevent him from hitting just prevents him from releasing frustrations that are perfectly natural and age-appropriate.
Here is a link to an article I wrote that also deals with this issue. http://www.gilabrown.com/GB/Blog/Entries/2009/7/21_The_Ga...
I hope this is helpful. If I can be of any further assistance, feel free to contact me.
Be well,
G. B., M.A.
Child Development Specialist & Parent Educator
www.GilaBrown.com