I define shounen as a type of manga about character(s) that fight/compete and constantly grow stronger* while facing stronger enemies.
DN, there are no powerups for Light, unless you count the DN itself. There are no stronger enemies either. He doesn't get stronger/smarter, but he does grow as a character, just not in the typical way.
As for Bakuman, I feel like it's not really a shounen either. I will admit it's pretty similar to a battle shounen, but there's just something about it that doesn't fit the bill. They described themselves best: a non-mainstream battle manga with non-mainstream protagonists engaging in mainstream fights while using non-mainstream powers.