I think my son thinks I fronted a some band called nirvana in the 1990s
How did your experience fronting Nirvana go?
Very well, I am so lucky.
Your son thinking you fronted Nirvana is amusing and it can be fun to have family jokes. That said, you should be honest with him so he can be in on the joke too and realize how comical it was.
I don't know I kinda like it when people think I'm the great Kurt Cobain.
I understand that it can be flattering for people to think you were someone famous.  However, when they discover you're not who you said you are they'll be angry.  Honestly is definitely the better policy here.