Don't let your son spend so much time on the computer that it becomes his major play activity. He should be playing with toy cars, blocks, and dress-up clothes. He should be pretending to be a pilot or a cook. He should be learning how to play alone and with others. He should also be talking and playing with you. And he should be playing outside, too, learning about flowers and rocks and bugs.
You need to understand that kindergarten teachers are not expecting children to be able to read on the day they start school. What teachers want is children who are ready to learn to read, count, and write through the development of solid intellectual, social, emotional, and physical skills. Too much time on the computer could limit your son's development of these skills.
You can learn a lot more about the importance of play in the development of young children by visiting the National Association for the Education of Young Children website.