"What I'm learning — and what's interesting — is that my wife has changed completely, and I'm sure that I have too," the actor, 35, told People.
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"She sees it in me, and I see it in her, and we're stronger now because of it."
The American Pie star and his actress-author wife Jenny Mollen, 34, welcomed son Sid into the world on 15 February. And there is no doubt that he has transformed their lives.
"The change that happens is so tangible. It happens overnight, and it's the craziest thing," Jason added, before deadpanning, "Otherwise, I don't think I've changed, it's too soon.
Jason and Jenny welcomed son Sid on 15 February
"Ask me again in a couple of years when the kid is milking me, and it's just horrible, and he's running around and it's exhausting."
Not that becoming a father has changed Jason completely — he is still addicted to reality TV.
"What I've learned from fatherhood is that having a son cannot, did not, change my love for The Bachelor," he admitted with a laugh.
"I thought that having a son would make me grow up when it came to my TV habits, but I love The Bachelor even more after having a child."
The star says becoming a father hasn't totally changed him