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In a lengthy post that begins, "I am the luckiest man on earth!" he talks about fatherhood and his son Shai being the "ultimate present" on his birthday.
"I'm definitely a dad now, but I don't feel older. On the contrary, I now have a bigger purpose in life than ever before and I feel limitless because of it," he added about his newborn son. "Parents always say 'you'll understand when you have your own kids' ... I now understand."
The dancer's fiancee posted a message on Instagram praising his parenting skills.
"And to the best Father in the world Happy Birthday," Peta Murgatroyd wrote. "Shai is so fortunate to have you as his guardian and protector for life. I see you with him and cry with the amount of love that is in your eyes. Our love for you is for eternity, forever the 3 of us are together."
The two "DWTS" pros welcomed their son on Jan. 4. They announced their engagement in December 2015 and then revealed they were expecting six months later.