Hi S.,
Yous story sounds so familiar to me. My son (only child at that time) was not interested in playing with other kids at all and completely hated Parties. He would opt to stay home, even when I took him to Gymboree he played alone and would not sit under the parachute circle time thingy they had. I finally stop going because he was not socializing and I wcould for no money take him to the park.
He was mostly with adults as I am the eldest in my family and no other grandchildren were around. I use to do the same thing as you, book him on play dates only to see him retreat to a corner or only play with me and ignore the other child completely. This continued until he began Pre-school, he also was very bright and a lot more mature than most kids his age..also physically he has always towered over most kids, which did not help matters.
I can still remember the first teachers conference, the teacher began with telling me my son was extremely bright and education wise he was far beyond pre-school. I was smiling from ear to ear, then she told me his social skills were a big concerned because he had none!! Apparently he played alone all the time and would only speak to the girls on occassions stayed clear from the boys and was essentially a loner...
Can't tell you how my heart broke on that one!! Anyway at that time he was going to school 2 days 1/2 a day....The teacher recommended I increase the number of days he came to school so he would have consecutive days. That made the difference within a month he was talking about his friends and on my next Teacher's conference I was told he had two friends that were boys and his Social skills were improving very much.
Now at 6 years old he is still very conservative , he is not the type to walk up to any child and introduce himself, however, he warms up much faster and eventually becomes everyone's best friend. On my last teachers conference the teacher commented on how he was by far one of the most popular boy with the girls, for his warm, kind and gentle ways and with the boys because of his height, strength and intelligence.
You have a shy boy who with the right encouragement will blossom to be the kid everyone wants to talk too.
I would only worry if crying or bouts of depression are noticed to preceed playdates...
However, it sounds like he just needs more time with kids and more time to learn the social skills...socializing with another child of similar temperament may help as I always felt my son was intimidated by the more outgoing kids....he did tolerate better kids who were also shy...