Faris gave birth to little Jack on 25 August (12) - more than two months before his expected arrival date - and the tot had to be monitored by doctors in a Los Angeles hospital before the couple could take him home.
Pratt now insists the four-month-old is healthy and hitting major milestones - like learning how to laugh.
Pratt tells U.S. talk show host David Letterman, ""Oh, God, he is just... he's amazing. I love him so much. He's just a ball of joy. He's terrific.
""He was born nine weeks premature, so we spent a month in the intensive care unit with him. But he's totally healthy now. He is just so perfect and I love him so much.""
Lost In Translation star Faris married Pratt in 2009 and they announced they were expecting the baby last May (12).