Not much point trying to explain why he shouldn't do what he's doing, because psychologically he isn't capable of sympathising with others.  CHildren only develop that skill when they reach 7 to 8 years old.  Before that, they are primarily egotistical.  You can only deal with it by disciplining the bad behaviour.  Warning then Timeouts.  There are a muber of articles on MINTI about discipline.

My advivce is you should exclude your son when he behaves badly.  Social punishment is the fairest and best form of discipline. If you are not using this technique, you are probably being too soft.  I would discipline rough behaviour immediately with a warning and then a time out.  I have a 6 year old son and a 2 year old daughter, so I know and deal with the scenario everyday.

Read the article on Minti and elsewhere about discipline and timeouts and I am sure you can sort out the behaviour.  Just remember that he won't understand why it's wrong yet.  Just make him understand that if he does it, he will be disciplined.

Wombat

I have 4 yo boy and 1 yo girl. There were times that my boy played a little bit too rought to my girl. When the warning to him didn't work, I'd talk to him and said:
A. 'is your sister a toy?', he'd answered 'no'. and I would explain to him why he should be gentle to her, 'Your sister is not a toy, if you play too rough to her, she might get hurt or even worse she might break her  bone and if that happened we couldn't fix her. She's not like a toy that we could fix or buy a new one at the shop'  or,
B. sometimes I would say 'Would you like if someone play rough (or do the things that you did to your sister to you) with you?' and he would say 'No'. Then I would say 'so, why did you play rough with your sister?', he would say 'I don't know'. Then I'll explain, 'We don't want your sister to grow up to be a rough girl, so how if you play gentle to her so she would play nice to you too.'

I'm sure you knew he probably done those just to seek more attention from you. Good luck!