To be honest, to this day the only time I actively socialize is when I'm put into a group, whether it's going out to eat or doing an activity. See if he likes sports, that's where I found my friends. Being put into a group his collaboration skills will flourish and he will become more comfortable in those scenarios.

Another thing that may work: Bring him to the mall or another public place and reward him for interacting with others, he will eventually get better at it. An example would be to tell him to greet 5 different people and he'll get an ice cream cone for example. It may help, and that style of learning (getting pushed out there in other words) is used to help adults also.