I know this is an issue some people carry with them into their forties, which, as disconcerting as that is, is something I don't think will evolve with him. But what I worry about is the hunt for a soulmate. The one friend, the best friend, who you know you can turn to over your entire relationship and he or she will be there arms open. Someone who gives as much to the relationship as you do, and is able to reap as many benefits from your shared time together.
He appears to be on a hunt for someone like that. He brings up that other children have siblings from time to time and he feels like they always have a playmate. I think he sees enough playmates and friends throughout the week, but he doesn't seem to like to play alone any more -- not even for a moment. And he used to not care at all.
The hard part is that last week he hugged me and said, "Mommy, you're my best friend." I had to tell him that I loved him, too, but that I wasn't his best friend. I was his Mommy, and that I was going to make some decisions that he would find wildly unpopular from time to time that would confirm that I was not a friend. He looked at me quizzically and then said, "Daddy, is your best friend. I need to find a Daddy friend."