Aggression

My son, who is 7, has recently started being aggressive with other children.  About two weeks ago, he hit a playmate because the other child would not get off the computer at before care.  His punishemnt was that he could not go outside after school and he has not been allowed to go on the computer at before care.  We are working on a reward system with stickers. If he can behave at before care for five days then he can go on the computer for 20 minutes.  My son determined what he wanted for the reward. 

Yesterday, he hit a child with a stick.  He told me that they were playing around and it was an accident.  I called the other child's mom and she said the other boy was ok.  I spoke with my son and he understood that he should not be playing with sticks.  Well, before he went out to play today, I spoke with him regarding the "rules".  We went over how to keep your hands to yourself, how to play nice with the other kids, etc.  I guess a little girl took his shoe so he punched her in the nose.   I made him apologize to the little girl then he came into the house for the rest of the day.  I am not sure how to discipline him for these latest actions.  I sometimes feel as though he is always punished.  Any thoughts?