I have tried telling him how much it hurts me, but by that stage he is too upset to listen. Sometimes if I can see its a deliberate one he is sent to his time out chair. He is distraught when he does it so I guess its good he has empathy. I know its his under developed brain and so know we just have to ride it out.
Time-outs are not effective and may be the cause of the frustration he is feeling. Gentle parenting, if I understand it correctly, is about neither rewards, nor punishments. A time-out is a form of punishment. I'm a father of a one-year-old, so I might be wrong about this, but what I would try to do is understand why he's hitting, then help him get over whatever it is.