The 'Backroad Song' singer added: “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.”
River's mum, Amber, 37, also shared the heartbreaking news on her Instagram, alongside a photograph of her holding her little boy.
"In lieu of flowers or gifts, please send donations to Dell Children’s Medical Center in River’s name. The doctors, nurses and staff have been incredible," Amber asked her followers.
Country music star Granger and Amber, married in 2010, and welcomed their daughter, London, one year later. She became a big sister when Lincoln, now 5, was born in 2014, and River followed in 2016.