If he senses that this worries you, he may try to stop doing it or may hide his imagination and that would be a pity and a shame.
Why is it an issue for you? Is the concern that he's mostly playing these imaginative games alone? Would you be fine with it if he were doing the very same things with other kids? I suspect you would and that the issue is that he's doing it in his own head and not worrying about involving other kids. Is that right?
He sounds great. I don't think the imagination is a sign of "social immaturity" necessarily; have you talked with his teachers about whether he is drifting off in class? Is he talking back or being distracted? Doesn't sound like it. Is he otherwise socially fine in class time? If he is relating to peers in the classroom, in gym, in group settings, when out among people with you, and that's all OK, please don't worry about his wanting to play this way and on his own; he may need the mental "down time" during recess and after school. He may be recharging his batteries by not playing with other kids with whom he's spending every single minute of his school day.
At eight, he is a perfect age to start some kids' drama classes or even get involved with a kids' theatre. Around our area there are lots of small kids' drama and music schools and even a theatre that is entirely for kids, by kids. Not professional stuff! No pressure, just fun. There are even one-day workshops for kids about acting. See if this would interest him and give him an outlet where he shares his imagining more. But please don't look on it as a way to make him socialize with others.
He might end up being a writer or comic book artist or actor or teacher who is able to reach out to others because he's imaginative. Unless his imagination is negatively affecting his schoolwork (which it seems it isn't) or causing him to have social issues in class or problems with misbehaving in class, church, events, or other settings -- I'd leave him be. Please don't think of the imaginativeness, or the strong focus on the favorite movie or character of the moment, as "social immaturity." As for limiting it -- please believe me that this is going to pass, if you're sick of Superman right now. You'll be sick of some other character next. I would question why he even needs to be "redirected" or his imaginative play limited.
Does he have one on one play dates at home? That could be great for him, but please don't turn those into "You guys need to stop playing Superman now" - let him and the other kid work it out. Worry less about what other kids think of him on the playground or about whether he'll be labeled. He's smart, he does well at school, let him have this part of himself without redirection or limits. If this were affecting him at school or meaning he couldn't function or do what he's told at home, that would be different but if it's just your own fear of labeling -- that's your own fear, not his.