As the father of a lot of now mostly grown up children, it's been my experience that they all do things at their own rates, including develop their social lives. I have some real party animals and my one lad who although he has 2 friends didn't have a girlfriend until he was 21- and to be honest you oculd haveknocked me down with a feather when he turned up with her ( never eally went out, always played computer games etc).
I'd give him a little bit of slcak on it to be honest and let him find his own way a bit or else just encourage his younger brother to include him in as much as possible, but I'm fairly sure he'll find his feet in his own time.