I'm worried about my twin brother?

I'm very concerned about my twin brother. He's always isolating himself and being on his own, even at uni at lunch he's sitting at the cafeteria stands by himself eating his lunch. I'd sometimes see him and go and talk to him and ask him why he's on his own and he'll reply because "I don't like being disturbed" or something like, he doesn't talk much anymore either. I've introduced him to nearly all my friends and my girlfriend also but he never seems to put in the effort to keep in touch with anyone not even his friends. When I'm going out on the weekends I'd try to invite him to come along but he always declines and spends most of his time on the computer, doing homework or the XBOX, it's like he has no outside life. We're 23 turning 24 in aug and I worry about him not ever finding a girlfriend or enjoying life the way I have. I've told him this several times and he'd reply "relationships are a waste of time" or some stupid excuse. Every effort I've made for him, he'd just turn his back on me or sometimes even snap like doesn't like to be around anyone not even our family. I guess what I'm trying to say is I wish he could see there's more to life then being on your own and playing video games all the time and we're not getting any younger so I don't want him to realize this when he's like 30. What should I do? how can I make him realize this?