American Idol alum Kris Allen says his 3-year-old son is "a good big brother" to Rosie

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Credit: Jason Myers

While Kris Allen initially expressed some concerns over how 3-year-old son Oliver Neil would react to sharing his parents with a new sibling, the toddler quickly took to his little sister Rosie.

“When Rosie was a week old, my wife [Katy] and I had some friends over,” Allen, 31, says in the new issue of PEOPLE. “They have a little girl, and she was going up to Rosie and touching her. So my son goes over there and says, ‘Don’t touch my little sister!’ He’s protective of her; he’s a good big brother.”

Allen and his wife welcomed their second child, Rose “Rosie” Elizabeth,” in June, and the American Idol alum has found raising a daughter is different.

“My son is my little buddy, but I want to keep him in line and make sure he becomes a man someday,” he says. With Rosie, “She’s just got me wrapped around her little finger.”

While Oliver and his little sister have seemingly become fast friends, they apparently don’t share musical preferences.

“When my son was little, I would play guitar around him, and he’d go, ‘Stop, Daddy, that’s too loud!’ ” says the singer, who will hit the road this winter for the Comfort & Joy Tour, on which he’ll sing tunes off his upcoming Somethin’ About Christmas album (due Nov. 4).

“But if I play Rosie a song, she calms down, and she smiles. She has a little bit more of a love for it.”

For more on Kris Allen, pick up the latest issue of PEOPLE, on newsstands now.