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katharine mcphee/instagram Katharine McPhee with son Rennie
In one of the photos, McPhee is holding Rennie as she waves at the camera from a trail in a sunny garden. Another photo showed the Smash star giving Rennie a big smooch on the cheek in front of a body of water.
"HOT MOM!!!!! 🔥 " Foster, 71, commented on the post.
McPhee and Foster welcomed Rennie in February after tying the knot in June 2019.
The American Idol alum said in April that being a mom is the "greatest job" she will ever have.
"He's such a good little baby and I'm so in love," she told Kelly Clarkson during an appearance on the singer's talk show.
katharine mcphee/instagram Katharine McPhee and son Rennie
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In July, she told PEOPLE that Rennie is "advanced" in his developmental milestones.
"His little teeth popped out and it's fun because they popped up earlier than when they were supposed to. He's a much bigger baby than his peers, so it's kind of fun to have a baby who is growing so well," she said.
SplashNews.com Katharine McPhee and David Foster
"It all evens out," McPhee added at the time. "They all end up in the same place, but right now he's a little bit ahead of the curve. It's super fun to be like, 'Oh, he's advanced.' But really you know that your baby is just the same as any other baby."
Last month, McPhee shared a sweet picture of her husband spending time with Rennie on her Instagram Story. She wrote alongside the candid shot, "My boys."
Foster is "always walking into a room saying, 'Where's my baby? Where's our baby?" McPhee told PEOPLE back in May. "He wants to hold the baby all the time, and be around the baby."