My impression of that the boy is that he is a timid person. Though the children in the playground were playing and having a great time he remained demure. In my point of view I think that he also wants to play with the children as he was feeling bored to his bone expressed in the phrase "And other boredom-beating tricks". He is an antisocial person ans thinks that the other children would not accept him into their clan. In contrast on the other hand the children did not take the initiative to ebrace him into their group.