In January, actor Gary Sinise lost his son, McCanna "Mac" Sinise, to cancer. He was 33.

Now, Sinise is describing in detail Mac's final days, explaining that in the week before he died, he finished recording an album that he'd been working on for the last year.

"I bought him his first drum set when he was 9 years old," he remembered in a conversation with ET. "He played through high school, and then he went to USC Thornton School of Music. He was playing drums all through college and studying songwriting and composition. He created a bunch of pieces of music while he was at college."

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Sinise added, "He just put them away and didn't do anything with them. About January 2023, he said, 'Dad, I think I want to try to finish this music that I worked on all those years ago in college.'"

Mac played the harmonica, having learned it during his cancer battle. Sinise said that the album is currently being pressed and will be available for purchase on the Gary Sinise Foundation website, and that soon it will be available for streaming as well.

"That last week, he finished the record," he said. "They had a viewing party... It was a joyful day. The following night, I went on television and talked about Mac. Then the next day, I had to call the ambulance and take him to the hospital."

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The routine wasn't anything new for the family — Sinise said that his son had been sent to the hospital, stabilized then released "so many times." He said that he assumed that this trip would be another one of those occasions, admitting, "I wasn't thinking that this was going to be the time."

In the last couple of days of Mac's life, he did realize that the end was near, but he said that Mac "was happy that he'd accomplished what he wanted. That's what I want people to know about... Mac never gave up. He kept fighting."

A time came when the fighting got "too tough," with Sinise saying, "It wasn't going to get better. It was going to get harder." Still, "He was at peace. He was happy at the end of his life. He was joyful... He was smiling."

The actor shared a story, recalling, "I was staying with him at the hospital at the end... I had to step out for something at one point, and I stopped by the nurse's station on the way in. I said, 'How's Mac doing? What's he's doing?' [The nurse said], 'He's watching "Forrest Gump" on his phone.' He was in there watching ‘Forrest Gump.’ It made him feel good. I think it made him feel closer to me sometimes. He would watch it when I was gone."

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Sinise said that in 2018, Mac began experiencing a significant pain in his tailbone. He wrote it off at first, but when the pain persisted, he went to see a doctor who discovered a "massive tumor" on his sacrum, a bone in the lower back. The tumor was removed, but the next year it came back.

Doctors kept removing the tumors, but more kept growing. As time went on, he remembered Mac getting "more and more seriously ill," but said that he continued to fight the illness.

"It was not going to get better. There's no cure," he said. "We tried 25 different drugs. These are just drugs that are used on other cancers and the doctors kept trying and they said, 'As long as you want to keep trying, Mac, we'll keep trying. We'll come up with something.' Mac said, 'I want to keep trying.' He was an incredible fighter."

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Eventually, the cancer paralyzed him, something that Sinise said was, in a way, a "blessing."

He shared, "It was very painful and all of a sudden he couldn't feel anything from the chest down, so that pain that he was feeling before was gone now, but he couldn't walk. It's just the nature of this awful cancer. He was fighting and I wasn't going to stop fighting for him. I never wanted to think that we were going to lose the battle even though you know that you're fighting uphill all the way."

While he and his family want to share their story, he confessed that it's "hard to say" exactly what that looks like right now.

"It's hard to even think like that, because somehow it feels like he's just on vacation, he's gonna come back."