April 10, 2016 11:15 AM

Carrie Underwood has a lot to be thankful for, and her son Isaiah tops the list.

“He’s the best thing,” she told PEOPLE at the 22nd Annual Muhammad Ali Celebrity Fight Night in Phoenix, Arizona on Saturday. “I never really knew what it was like to love something or someone unconditionally until I had him.”

The “Church Bells” singer added, “He’s so sweet and so smart. He’s a little sponge just growing and learning every day. He’s just the best thing.”

Michael Tran/FilmMagic

Underwood, who welcomed her 13-month-old son with Nashville Predators star Mike Fisher last February, admitted that the little one — who “climbs all over everything” nowadays — is at the stage where he repeats everything.

“He’ll hear you say something and then he’ll mock you,” Underwood, 33, said of her son’s imitations. “Right now he walks around the bus and he’s like ‘OK, OK.’ It’s adorable.”

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The American Idol alum, who wed Fisher in 2010, said that though having a baby hasn’t entirely changed their marriage, it has forced them to become more aware of the time they do get to spend together.

“Sometimes you have to make more of an effort to be with each other because it’s really easy to put all of your time and energy into your baby,” explained Underwood. “It’s been — not challenging, but just something that we’ve really just trying to be mindful of and try to make time for each other. We’re planning a vacation right now — we need it!”

— Christina Dugan

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