My ds is generally happy in his own company. He has friends but on a functional basis, to watch football with, go cycling with, etc. He says he "doesn't do" small talk when I ask why he was quiet if we've had people over, or seen someone we know in town.
If people chat to him about sport, something in the news or work related chat he's fine, but simple "how are you" conversations are met by yes + no answers.
I'm quite sociable, + luckily so are both of our dc's!
If we're all invited to something together and it's pre-planned he'll come + make an effort, but spur of the moment things are, "You + the kids go, you'll enjoy it, it's not as if I've got anything in common with anyone there"!
I then have to make excuses as to why he's not there, or insist he comes + it's obvious he's not enjoying it!
He does a lot with the dc's, but these tend to be taking them to national trust places + art exhibitions etc. If I suggest he comes to a funfair he's suddenly got "things to do!"