No suggestions, because my DS10 is similar and I haven't figured out why or what to do about it, if anything. He's a very outgoing kid, but has absolutely no desire to sleep over at anyone else's house - he'd prefer to be at our house. He's been invited to go places with friends, but he prefers not to go *with* them - he'd rather we meet them there and (one of us stay as well).
He doesn't have a problem going to school or on field trips, etc., so I don't think it's a separation anxiety issue (although, I feel certain he'd *freak* if we told him he was going to an overnight camp a day away). My best guess is that he's just not comfortable being under the care of others and that's about what I can gather from asking him. Like your son, it's become more evident as he's gotten older - a friend and I used to swap kids pretty regularly for playdates and stuff when he was younger and he never gave it a second thought.
I don't see it as a huge problem now, other than making sure his friends (and their parents) don't think it's something against them if DS doesn't want to spend the night. However, I sometimes wonder if/when he'll outgrow it. Fortunately, none of his friends seem to mind being over here all the time.
