I think disciplining him when he gets home (no computer games) is a bit delayed for something that happens at pre-school. How are they coping with it at pre-K ? Obviously he is not hitting, pinching or yelling at home. So the problem is at preK, and not home. He should be disciplined at pre-K. I would talk with the teacher, and find out how she is handling it. Team up and make a plan, tell her how YOU would handle it if he did that at home, and have her handle it.
What behaviors preceed his acting out ? Is he being bullied? (he is the youngest, after all) Is there some way to intervene BEFORE his behavior turns ? Most of the time, it takes two to tango, so I doubt he's acting out in a vaccuum.
That said, my other question is: Has it been this way all year, or is this new Spring behavior? I ask because I remember when my daughter was just barely 5 and in K, and and I did notice that when Spring came, and the curriculum began to push a bit harder toward year end, she began to struggle some. If you have the opportunity, I would NOT send your son to Kindergarten at age 5. I have 2 girls, both born in August, who made the K- cut-off agewise. The older one we held back a year and we were REALLY glad we did. The younger one went to K at age 5, and I have often wished I had held her back so she would have had maturity on her side as she went thru the school system.
It's up to you, but the gift of an extra year of maturity can make a HUGE impact on your child, even if he's smart. It's not the grade level but the social issues that are hard to cope with in school. And the social issues tend to decrease with maturity vis a vis their peers.
Good luck !!
b