First of all...note the source.  Not super reliable.  

"The Secret" is that he's apparently hiding a "secret son" that isn't actually his biological son. He's actually the son of Tim's former fiancée, Kristine Donahue, Tyler Zarbo, who is now 23 years old.  

He said, quote, "Tim McGraw was like a father to me when I was growing up. He saw me as the son he never had, and we've stayed in contact.  I wish he could have remained as my substitute dad, but it just wasn't meant to be." 

The article goes on to say that Faith Hill is extremely jealous and will "jump all over him" if he even glances at another woman.  

Article here: National Enquirer