Hi,
Is the folowing sentence correct?
He bears the nickname of "the freak" because of his weird behavior?
Yes, but most people would omit "of" in this case.
And because it's a name, "Freak" should be capitalized.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.