I had a little guy like that as well. Tried everything to engage him in solitary and group play when the older kids left for school to no avail. He was sweet and pleasant, but was content alot of the times to watch the other kids. It bugged me too, but when I saw he was otherwise happy and chatty I let it go though I still always made a point of inviting him to play. He eventually came around and joined in, but was always in the follower role.