Should I be worried about my cousin?

Ok so I'm 13 and my cousin is 18. He grew up in Australia, and then came to my country to serve the army since he was born here. He does come back from Australia every year, he has been serving the army for a month now and comes back every weekend. But it just seems that he is very antisocial. All he does is sit at the computer, play games, watching YouTube and gets served his daily meals all there. He never gets up from his seat unless he needs to go to the toilet or get a snack to eat. He was exactly the same way like this before he went to the army. He sleeps very late, past midnight using the computer and drinks a can or 2 of coke everyday. His parents and siblings are all in Australia, so my grandparents are the ones taking care of him. If we all have to go out for family dinner and my cousin doesn't want to go, we all can't go. My parents and I were initially invited at first, but I didn't want to because I knew that my cousin wouldn't want to go if I went. My parents didn't go because they were very tired. So my brother wanted to go, so he went to meet at my grandmother's house. When my grandfather asked my cousin whether he wanted to go, he said no. So in the end they couldn't go. It's like everything was already decided. Should I be worried?