Gary Sinise sits down with ET's Rachel Smith, and opens about his late son, Mac, and Mac's five-year journey with cancer. He shares how the health battle came only months after his wife, Moira, was diagnosed with stage 3 breast cancer, detailing how he navigated the difficult time. Gary also shares how he and Mac bonded over music, and why he decided to tell his son's story on The Gary Sinise Foundation website. Donations can be made to https://www.garysinisefoundation.org/.