The couple, who were married in July 2009, celebrated the birth of their first child in August 2012. Pratt said that although their son, Jack, is now healthy, his early delivery was frightening.
"We were scared for a long time," Pratt, 35, told PEOPLE on Wednesday. "We prayed a lot."
Jack spent one month in a neonatal intensive care unit while his parents and other family members prayed for him.
"It restored my faith in God, not that it needed to be restored, but it really redefined it," Pratt said. "The baby was so beautiful to us, and I look back at the photos of him and it must have been jarring for other people to come in and see him, but to us he was so beautiful and perfect."
Four months after the birth, Pratt reported that Jack was hitting his milestones and doing great.
"Oh, God, he is just ... he's amazing," he said on the "Late Show with David Letterman."
"I love him so much... He's just a ball of joy. He's terrific... He's totally healthy now. He is just so perfect and I love him so much."
Now, at nearly two years old, Jack's developing personality has Pratt smitten.
"He is such a fighter, he's amazing," he said. "He's so open and there's no fear in him no matter what.
"He is so charming that my plan is to just let him take care of us as soon as he's old enough."
Faris, star of CBS sitcom "Mom," considers her husband pretty charming as well.
"It's easy to assume he is sort of a golden retriever of a man, but he's really smart," she told PEOPLE. "He's a really loving father, he is an incredible husband. He doesn't let fame seduce him. He's still the same dude."