My boyfriend has a son who I refer to as spawn.
Why do you call him spawn?
Because he's the spawn of Satan.
It's disrespectful to call your boyfriend's son spawn, because that's not a very nice name. If you continue to call him that then your boyfriend might not want to be with you any longer.
I won't say spawn anymore. He's just a child and he sounds like a spawn.
That's good, you don't need to refer to people as rude names that they wouldn't like to be called. You could imagine how much it would hurt if you found out people had a rude name for you.