The country star told Today's Hoda Kotb that he found his birth certificate with Tug's name on it while going through his mother's closet.
"At 11 years old it was very confusing," Tim said of the discovery.
The singer said he didn't grow up with Tug around and that he started to get to know him better before he died of brain cancer in 2004 at age 59.
"It changed what I thought I could do with my life," Tim continued about the discovery.
"To me, it made me think that blood is in my veins. So that ability is in there, I found sort of that grit inside me that he must have had in order to succeed."
Tim will be a releasing a new book Tuesday that details his personal fitness transformation titled Grit & Grace: Train the Mind, Train the Body, Own Your Life.