I think this one is wrong, but I know a way to make it works
I am a good boy, aren't I?
You can say it's the same, but as a not native speaker I was told by my teahers that in Tag questions or Attached question, with "I am" you should use "aren't I" because there's no "amn't I"(well there is in Irish English but not in Standar English), so it is right saying "aren't I" in this case, but in the original sentence you gave, I'd say it's wrong, althought I'd choose saying "Ain't I a good boy?" but that's not totally right but it doesn't sound wrong either
If I'm wrong about tag questions, just let me know it so I can ask for my money back