First off, I would like to say that it's, more or less, an enjoyable story. It has great writing, great grammar, great flow—essentially a recipe to a good story.
What's not good about this story, however, is the story and characters.
I'm gonna be blunt. It just comes off as a wish fulfillment story. The main character has almost everything going perfect for him. And not only does everything go perfect for him, he IS the perfect person, amazingly intelligent, fit, nice etc. . . And don't get me even started on the system . . .
Please, when writing about events, talk about the conflicts. Talk about why it's hard, why it's bad, what went wrong. You need to provide a contrasting texture to the parts of the story where things go wrong. Through events where things go wrong, we can then get character development and an interesting story.
I'll illustrate what I mean a bit clearer using examples in the story. So, you have Jay stay in the rehab center (I think? Apologies if I'm using the wrong term). He decides to exercise every day. Great. Somehow, over six months he becomes incredibly and completely ripped due to his very rigorous exercise regime. Is this a thing that any person can do easily? No. It's a very unrealistic event, otherwise we'd see people like that every day. And doing it in six months is bordering on the absurd.
When writing about unrealistic events, you have to make it clear why this works and is realistic in your story. Talk about how hard it is for him every day, and why your character in particular is able to go through this; he has strong willpower, is goal oriented, or very motivated? If yes, why is that so? SHOW that through the story then—don't just tell us that he has those characteristics in a list and directly to the reader.
Then, try to illustrate through those events how much effort he puts into it. You basically sum up his entire effort in a few sentences AND he has no emotional reaction to achieving such a difficult thing. That's bad. He achieves what most human take years to do in six months and he doesn't seem to care (starting from ****** rehab). And I know you have the system as an excuse for this, but that simply makes for a cheap story (and still doesn't SHOW his emotional strengths in handling the pain). Don't use it as a crutch to explain away bad writing, but as a way to make your story better.
My other big issue is the character. Don't have him perfect. Please. When essentially everything he gets is through no effort that is shown (yes I repeat the word 'show' a lot—it's very important in writing) it feels very unjustified and cheap. Try to take away some things from him. Does he have to be 6'4", genius intelligent (no, graduating high school does definitely NOT make it easy to redo it 10 years later, contrary to what ******* writers seem to think . . . ), amazingly handsome, fluent in 6 languages and soon to be a great singer too? Slow down with these.
If you really plan on keeping them, then play with his personality. Give him weak points.
And, by weak points I don't mean trait-that-is-slightly-bad-and-has-no-effect-on-the-story—like, the weak points you might say in a job interview, "Oh yes, my one weakpoint is that I am too hardworking and dedicated to work". I mean, give him an actual honest to god weakness in personality. Is he a jerk to some people? Shy? Bad with relationships? Does he rub some people the wrong way due to something he does? Scammed easily? Gullible? It makes for a far more interesting story and characters.
Next, not everyone in the story has to actually like and enjoy the character. Some, might just not like the MC, his music, his appearance—just like in real life (see a pattern here? realism is good). This can then create interesting plot developments (e.g. a person who initially dislikes him forms a close bond with him => easy character development).
Finally, my last points. Conflicts. EVERY challenges the MC has had was easily solved. Pls no. Don't do that. You turned potentially very interesting plots points into an unrealistic glorification of the protagonist. A story doesn't necessarily need conflicts (post-modernist literature i.e.), however if you have a singer's bildungsroman, it's almost a necessity.
For example. Many conflicts your character has in the story are similar to the one he had with his two childhood friends (Cathy and Emmy? I forget). In fact, it was conflict caused by another person, not the MC. Ergo, the MC's only conflict was how to tell them that they are treating him like a child, while the two girls had an actual conflict: they realized their treatment of him was not how he liked (though you didn't actually do anything with this beyond one sentence . . .) What happened here? It was a conflict caused by a not MC, leading to not development of the MC. And this keeps repeating
Have the MC genuinely, completely screw up. Where a lot of writers make mistakes is that the MC's screw ups aren't really even screw ups—they are choices made by the MC that any reasonable person would have done too in the same situation, given the same information. In example: I screw up by giving accidentally poisoned water to my wife, completely unexpected given I have no enemies. Could I have saved her? Yes! Is it reasonable to assume that I'd check for poison all the time? No!
The problem with mary sue characters is, how do you make them convincingly screw up when they are literally perfect? Any conflict that would arise would feel unconvincing and out of character given their perfect personality. Hence why you need to give them good weak points.
I know, my review is quite harsh. It's simply that you write well, and I'm judging you by the standards of a good story—else I wouldn't bother with this cluster. I enjoy your story, it just really could be better in some regards.
