NTA. But with a concern.

While you’re 100% right that having an adoptive father offers your son protection. If you were to die tomorrow and children’s services was called, they would find that the child has no living legal guardian. Meaning someone has to become one, and that could be a crapshoot on if he gets to stay with his step-father or if his grandparents want to fight for it...it doesn’t sound like grandmother getting custody would be a positive thing.

I’m going to say that it is a bit of an asshole move to change his name at 10, though. Even if he’s on board with it now. I would wait until he’s at least near to adulthood before letting him decide to sever that tie or not.

I say this from experience - my cousin was in the same situation as your son. His father died when he was 6-7, and when his mother got married to her long time boyfriend, he adopted my cousin and they changed his last name. Turns out as great as the adoptive father was, my cousin feels way differently about it now and ended up changing his name back legally to the name he was born with, out of respect for his bio dad.