Hi D. W.,
I am an early childhood teacher, and I once taught an afternoon program of 5-year-olds (all boys!!). They had a remarkably large range of abilities and interests, so it took a lot of supervision, teaching, and coaching the kids to help them to achieve certain goals. For example, one boy couldn't pump and swing independently, when all the others could. I worked with him on this individually, and then he gained confidence in the group. Another boy retreated in frustration whenever we played soccer because he couldn't kick the ball. I worked with him on this on his own as much as possible.
What I am trying to say is that I think your son's behavior sounds fairly normal. He could be more of an observer right now because he doesn't feel confident in certain activities (bike riding, etc.). He sounds interested in participating and interacting socially, but holds back sometimes. This is very normal at kindergarten age. I think you can help him by reassuring him that he is a very capable and wonderful child, and you are proud of him, no matter what. It does not help a child to feel his parents are anxious about his development. Be confident in front of him, and give him confidence by reassuring him he is okay just being who he is. He will figure out how to play, and he might just need more time, or he might need a new group of friends. Maybe try play dates with other kids he likes from school.
At this age, I think it is VERY important that parents are involved in play dates and in observing their children's play. Too many adults assume kids can "work it out" themselves, but sometimes they are just not ready to do so. Once, I attended a party where I was the unofficial chaperone of a basement full of third graders. The parents were partying upstairs and were nowhere in sight. The girls in the group were saying really horrible and nasty things to each other, and getting physically aggressive, to the point where I had to intervene. Other, younger children were watching this bullying, and it was just not appropriate. Kids needs some supervision, and in your son's case, it is important to see that he is playing well with others and that he is not made to feel less capable and maybe there are fun games you can suggest for him and his friends to play. Anyway, stay involved, and make learning to play fun, and your son will do well, I am sure. Take care.
Nessa C.