"I can't imagine what y'all are going thru," Aldean wrote on Smith's Instagram page, where he shared the sad news. "Praying for u and ur family bother. We are heartbroken for u guys."
On Thursday, Smith, 39, canceled a show in Kansas City and shared an adorable photo of himself with his youngest son, River Kelly Smith. In the caption, Smith said his son died following a "tragic accident," but offered no further details.
"I have to deliver unthinkable news. We've lost our youngest son, River Kelly Smith," Smith wrote. "Following a tragic accident, and despite doctor's best efforts, he was unable to be revived. Amber and I made the decision to say our last goodbyes and donate his organs so that other children will be given a second chance at life. Our family is devastated and heartbroken, but we take solace in knowing he is with his Heavenly Father."
Smith continued, "Riv was special. Everyone that met him knew that immediately. The joy he brought to our lives cannot be expressed and his light will be forever in our hearts. If there are words to say more, I cannot find them in this moment. Love the ones close to you. There has never been a more difficult moment for us than this."
The "Backroad Song" singer asked fans to make a donation to the Dell Children's Medical Center of Central Texas in River's name in lieu of flowers.
Smith and his wife, Amber Emily Bartlett, also share son Lincoln, 5, and daughter London, 7.
Aldean was not the only country star to reach out to Smith and his family after hearing the heartbreaking news.
"Praying for my friend [Smith] and his family. Can't even begin to comprehend what they're having to face right now," Travis Denning wrote.
"Thoughts and prayers with [Granger Smith] and his family. Such a nice and humble guy every time I've ever been around him and you hate for anyone to be going through something like this, but especially someone like Granger," Brandy Clark tweeted.
"Thinking about you [Smith]. love you man. so sorry to hear what you are going through. my listeners feel the same. lots and lots of love coming from us," radio host Bobby Bones tweeted.
"Many prayers. So sorry," Luke Bryan wrote on Granger's Instagram page.
"Man I'm am so sorry to hear this for you and your family. You are such a great, hardworking, kind human being and I know how much your family means to you. Wow. I can't imagine what y'all are going through. Prayers to you and your family," Jake Owen added.
"I am so, so sorry," Maren Morris wrote. "Thinking of you and your family at this time."
Photo credits: Getty Images