Though it's been nearly two years since Gary Sinise's son lost his battle with cancer, the actor continues to carry on his legacy with the "treasure chest" of undiscovered music left behind. In an exclusive interview with Fox News Digital, the "Forrest Gump" alum, 70, opened up about his grief journey since Mac Sinise died from a rare form of cancer in 2024 and explained how he and his family plan to keep his memory alive. "Mac died on Jan. 5th, 2024, and actually the last year of his life was a pretty amazing year for him," Sinise recalled. "I think he was feeling like things were closing in on him. It was a very, very difficult cancer and there was no cure and none of the drugs were working, and he kept trying to try, but nothing was giving us any real hope. And so, I think he was very practical and very realistic about what was going on, and he had some music that he wanted to work on that he never finished."ACTOR GARY SINISE'S SON DEAD FOLLOWING BATTLE WITH RARE FORM OF CANCER"He was in pain," he continued. "Sometimes he couldn't sit up, it was just better to recline him back in his wheelchair because of some of the pain. But he had a smile on his face through the whole thing because he was listening to this music. He'd accomplished this wonderful thing that he started out with, and then I had to take him to the hospital within a day of that, and that was the end for him."In Feb. 2024, the Gary Sinise Foundation shared the news on Instagram of Mac's death, with the caption, "In Honor & Memory of McCanna ‘Mac’ Sinise 1990-2024." In a statement, the foundation said that on Aug. 8, 2018, Mac was diagnosed with a very rare cancer called chordoma. The foundation said chordoma is cancer that originates in the spine and only affects an average of 300 people in the U.S. per year. He was 33 years old at the time of his death. In Sept. 2018, Mac underwent surgery to remove the initial tumor, but his cancer returned by May 2019 and was spreading. "This began a long battle that disabled him more and more as time went on," Sinise wrote.Mac went on to have four additional spinal surgeries and continued to receive chemotherapy and radiation treatment. Sinise wrote that eventually the cancer caused Mac to become paralyzed from the chest down, though he "still had limited use of his right arm, and fingers on his left hand."His love for music remained steadfast until the day he died, Sinise said. "This is part of life. This is part of life where we lose loved ones," Sinise told Fox News Digital. "And for a parent to lose a child, it's really, it's a difficult thing to go through. I've met a lot of people, a lot of parents who've lost military sons and daughters over the years, and I've tried to help them through by providing services and support for them. I think I've learned quite a bit over the years watching these families deal with that kind of loss. I think that's helped me for sure."GARY SINISE SAYS VETERANS DAY IS A REMINDER THAT THE MISSION NEVER ENDS FOR THOSE WHO SERVESince Mac's death, Sinise said he has stumbled upon a "treasure chest" of melodies that his son left behind — all, Sinise believes, are little signs from above."The fact that Mac left all this beautiful music and that I discovered it, so much of it I hadn't known about. He just wrote these things and kind of tucked them away," he said. "I thought there couldn't possibly be more music after I did the second album," he added, later saying, "I found so many things, and it's like Mac just built this treasure chest of beautiful melodies."Last year, Sinise released his son's posthumous album, "Resurrection & Revival: Part 2." The album began shipping on Nov. 10, 2024, on what would have been Mac's 34th birthday, to fans who preordered the album.WATCH: GARY SINISE FINDS HEALING IN HIS LATE SON’S MUSIC, SAYS HE LEFT A ‘TREASURE CHEST’ OF SIGNS BEHINDThese days, Sinise said he's working on the third part of "Resurrection & Revival.""He wanted all the proceeds to go from the vinyl sales to go to the foundation to help our mission, to help our veterans and everything, so he continues to support," he said. "He did a lot of great work at the foundation and left a lasting impression, a great legacy of service, and this music is tremendous."