Wrapping it up
There are some key components that your story is missing. While this is a fantastic concept, I'm not sure that the way this story is presented is quite realistic. Here you have this nerdy, dweeby character who has been relentlessly bullied his whole life. His level of apprehension, if not outright fear, should be much greater than than he displays whenever Marty first appears in his bedroom, for starters. He kind of just accepts everything Marty says immediately. There's very little confusion or doubt, and a complete lack of fear. This seems completely unrealistic, both because of how people would generally react and most especially because of the personality of your character. As someone who has been pushed around and beaten almost daily, you think he would be especially weary of intruders. That, and the fact that with everything Marty told him, Mike's answer essentially amounted to, "Alright, that's cool." Not only does this not strike me as a normal reaction, but you're missing a golden opportunity for inner-conflict and self-exploration. Don't be afraid to give your character conflicts.
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Another suggestion would be to add more character and setting descriptions. You have very little, which can become a problem as descriptions are what bring a world to life before a reader's eyes, as much as actions. It will also become crucial for the development of the story when explaining the difference between worlds.