Catherine, my DS3y6m has just recently (within the last six months) become more social with other children. Before, he would engage in "parallel play," much as you've indicated that Graham has. I believe that it's normal, and in a few months, he may wish to play with others. As for the obsession, while my DS hasn't been obsessed to the same degree, he has gone through phases where he wants to learn everything about bees, dinasours, etc. He LOVES trains and knows each Thomas character intimately. When he plays with his trains, they have to be in a certain order. If you change it up, he will glance over and "correct" the change to his preconceived notion of the correct order. I would give a strong second to Kriston's advice.
