But....everything on Mamapedia is "on a personal level." And please consider that requests for more information are not about prying or about "psychoanalyzing" you but about trying to give you information that might be more specific, relevant and useful if those of us posting know a bit more about how "mopey and emotional" manifest themselves in your kid. Those words can mean just typical preteen blahs or they can be signs of serious problems, or many things in between, but it all depends on the kind of detail you don't want to give.
So speaking generically, and yes, I do have a tween though she is not especially mopey or negative: Does your son have plenty to do, and is at least some of it what he wants and chooses to be doing? Does he have activities both in and especially outside school that truly engage him -- activities he chose, not ones that he was told he'd do, or ones that he's done since he was much younger? Most kids need something either to blow off steam, or to focus their attention, or both, and it needs to be -- especially by the tween/middle school years -- something the child chooses and feels strongly about.
If you say, yes, he has activities but is still mopey, I'd ask myself if I were you: Are these activities the one's he's been doing for years now? Did we ever check in with him to say, "Hey, you've been doing X (playing soccer/playing other sport/doing church youth group/in speech club/whatever) for a few years now. How is it really going? Do you want to continue or do you want to try something new?" And so on. I've seen kids keep grinding out the same activities when they really needed a change, or the activities were ones that mom and dad put them in at a younger age but now they are passionate about....cartooning, or music, or writing, or a different sport, or Boy Scouting (which at older levels does many very cool things)....and so on.
Kids need something that is all their own; that takes them outside themselves and helps them interact with other kids and positive adults; and that requires them to keep at it. If your son has no or few activities, please think about working with him to find something that he is enthusiastic about and have him keep at it for a while. This might require some outside-the-box thinking -- for instance, if he's not into the sports that are typical, he might get into individual sports like golf, fencing, tennis, or if he's more about academics, he might want to try math club or Science Olympiad or other extracurriculars where he gets to be with other academically minded kids. Involving him in choices and asking him about what he wants to do are good ways to help him feel like he has more control; many tweens feel they are ready for some control and choices but feel they don't get them.
Volunteering at something that interests him and helps others is also good for getting a mopey kid outside his own head. Kids this age have a hard time seeing that other people have problems too; some volunteering could help him get past that and make him feel less like he "hates the world" if he sees that he can help change the world a little bit.