Not a question to diss the introverts as introverts are usually sensitive people who internalize their feelings, great observer and analyst. However real world can get tough for them as they often have to put up an outgoing front just to "fit in and survive"
They often need to take time off to be alone to recharge their energy.
Some could turn into escapist.
Anyway way you can help an introvert to unwind and to include you to be part of their life more?
To get their trust, you need to play the introvert aswell and get along with their thoughts and actions, only after that you gradually suggest other things. You can't come up to an introvert and let him know that you are gonna change his way of being. That scares people. Everything has to be gradual. But most of all, you can't feel you are changing your whole life to adapt to the other person. If you feel like you're doing a great effort to acept the way he is, you're the one who is gonna be incomplete/unsatisfyed cause you're changing who you are. And he will probably sense that too: that he is making you feel bad, cause you had to fit his way of living.
Man let me tell you, it is not easy... I have to put on a front all the time because I'm in the culinary industry. I was literally shaking the first day I had to greet customers. Like you have to be completely comfortable in your environment and at least try to get along with everyone around them, just seem friendly. Go out and attempt to be social, it's really hard and everyone is making you uneasy... Sometimes you won't notice you've been going out so much. You just have to keep going at it. Eventually you'll get used to it, and try to build confidence.
You shouldn't say "Man Up", but to help an introvert you have to start with what they're comfortably with. You can't force somebody to socialize if they don't want to. Some introverts including me are like that because they like to be alone. If he wants to socialize then help him by starting slow with going to the movies or the park and slowly increase.. :)