My husband is a hermit!?

I need help! My husband is very shy. Not with me, obviously, but when it comes to being around other people he will not say 2 words and acts just miserable. When we first met he was going out with his friends all the time and now he hardly has any friends and absolutely refuses for us to get involved in anything with other couples or anything! We always end up getting into a huge fight if anything comes up that involves people coming to our house or going to theirs. I don't understand. I know he's a homebody and prefers to be on the couch watching tv, but come on everybody has to want friends at some point. What can I do to get him out of this shell???