January 03, 2013 01:00 PM

CL/Splash News Online

Chris Pratt is one proud papa.

Since welcoming his first child with wife, actress Anna Faris, the actor has been busy basking in baby bliss with his son Jack.

“Oh, God, he is just … he’s amazing. I love him so much,” the new dad, 33, raved during a Wednesday appearance on Late Show with David Letterman. “He’s just a ball of joy. He’s terrific.”

The 4-month-old has been hitting his milestones — he recently started laughing, shares the Zero Dark Thirty star — which is reason to celebrate as baby boy has completely bounced back after being born over two months early.

“He was born nine weeks premature, so we spent a month in the intensive care unit with him,” Pratt says of his son, who was named after Faris’ father.

“But he’s totally healthy now. He is just so perfect and I love him so much.”

— Anya Leon

Bonus: Pratt shows Conan O’Brien how Jack does his exercises (skip to 3:15):

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