The veteran actor stars in the hit faith-based film, I Can Only Imagine

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Dennis Quaid is a grateful man.

With a career spanning more than four decades in Hollywood, the actor, 63, is now starring in the the new faith-based film I Can Only Imagine, which made $17 million its opening weekend. Quaid plays the abusive father of Bart Millard, the lead singer of the Christian rock group MercyMe, whose song of the same title was the best-selling Christian single of all time.

But happiness for Quaid isn’t measured in ticket sales. “That my kids are well. My kids are doing all right, that gives me the most peace,” the actor tells PEOPLE in this week’s issue.

For more of Quaid’s interview, pick up this week’s issue of PEOPLE on newsstands Friday

In 2007, the actor and his third wife, real estate agent Kimberly Buffington, 36, had twins, Thomas and Zoe, via surrogate (Quaid also has a son, Jack, now 25, from his second marriage to Meg Ryan). Days after they were born, the babies developed staph infections and were admitted to the hospital. While being treated, they were accidentally given 10,000 units of Heparin blood thinner—twice—rather than the 10 units prescribed. The overdose nearly cost them their lives.

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Dennis Quaid.
| Credit: Randy Harris

Now 10 years old, the twins are thriving. “My favorite time is actually when I’m in the car taking them to school,” the actor says. “During breakfast, getting them up, you really get to know them. They’re as fresh as they’re going to get. It’s not the witching hour at night when it’s time to go to bed and all the excuses come out. A lot of bonding goes on in the morning, talking about life.”

As for Jack, who is an up-and-coming actor, Dennis is “so proud of him,” he says. “When he said he was ready to act, I told him I’d help him out, and of course he’s Meg Ryan’s son, but he said, ‘No, I want to do it myself.'”

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Jack won roles in the 2012 blockbuster Hunger Games and HBO’s Vinyl and has just signed on to star in Amazon’s superhero series The Boys. “Now I’m like, ‘Hey, how about a little help?'” Quaid jokes. “I’m not too proud to ask.”

Becoming a father changed everything for the actor, he insists. “It took my focus off myself, which is always good for an actor because actors are so self-involved,” he says. “And I’ve learned more patience. Hopefully!”