Most middle schoolers don't curse. They know the words, but they don't dare use them. Use increases with age.
That said, Will Smith's son cursed pretty young cause his daddy did so often. Will had to explain to his kindergartner that these weren't school words. (This came from an interview with him that I saw.)
So why does this character think this method of speaking is okay? How do others react to it? Is he going for the shock value? At what point does that wear off?
My middle grade character curses, once, but his mother is dying dammit. The word darn just won't do. And mom calls him on it. It's a major scene in the book. It has impact. I'll fight for that word in that scene. Otherwise, I use the substitute words we all use: frig, crap, darn, drat. No need to make up your own. Remember, even in your book, the shock will wear off. The words will lose impact.
YA characters curse among themselves just like young adults do. It's common to the age group.