C.K.
He sounds a bit like my little guy who is now almost 6. I think at that age it is still "normal" for them to parallel play, rather than play together. Granted some children are better at playing together at this age than others - mine was not. My boy is smart beyond his years, and even now, although he has friends at school is still somewhat of a loner. He finds it better talking with older children and is not a rough and tumble little boy, although he is starting to "catch up" in that department. It could also be a personality thing with your little man - genetics is a wonderous thing - can you or your significant other think of how you were growing up? Unfortunately we can't always choose the traits we pass on to the next generation, and I have to say, some of the things I see in my boy I have experienced - thankfully he is not as painfully shy as I was!! Yes, I think you are on to something with the "stress" angle too, you probably just have to reassure him and include him and his brother in "unimportant" decisions if you know what I mean. Sounds like you are doing everything right though Mom!