Can I be prosecuted for unknowingly having sex with a minor who lied about his age?

Yes. What you are asking about is the defense of "Ignorance or mistake of fact or law".

However, it is not a defense to the crime of unlawful sexual activity with a minor, sexual abuse of a minor, or an attempt to commit either of these offenses, that the actor (you) mistakenly believed the victim to be 16 years of age or older at the time of the alleged offense or was unaware of the victim's true age.

Hope this helps.