This is more a question of audience than voice. You want to pitch the story as middle grade fiction, but how many middle graders want to read about a mid-20-year-old? Or a 15-, 18-, 20-, 22-year-old? Will the take-away from the protagonist’s long character arc resonate more with a tween or an adult? He’s living through several developmental stages, each with a distinct sensibility and concerns. Crossover readers aged 18-44 do read MG, but they aren’t the primary readership. I suspect this story is better crafted for the adult or new adult markets, with grown readers in mind. You can start with that youthful MC, but it’s worth experimenting with an opening that allows readers to meet and connect with the older protagonist first. A flashback approach could show his younger self. Or, you could start with that 7-year-old and a more mature voice, hinting that there’s an older presence looking back. Your first step, though, is to definitively identify your target reader. Answer this: If you sat at a table and started telling this story, who would be sitting on the other side of that table?