Grayson's mother, Michelle Arroyo, shared the heartbreaking news on Instagram Monday evening.
She posted a sweet photo of Grayson — who was first diagnosed with cancer in 2006 — and revealed that he died on Sunday.
"It was very unexpected as last week he had CSF [cerebrospinal fluid] drawn from his shunts and they adjusted the valve setting of his shunt and he was doing much better," she continued.
She added that he was "still struggling" with other health concerns when his heart stopped while at home.
"I performed chest compressions to get it going until the paramedics arrived and took over," wrote Michelle, noting the rollercoaster of events she experienced before Grayson took his final breath.
"When he left for the hospital he looked good and was breathing on his own but his heart gave out again at the hospital," she said.
Gretchen Christine Rossi/Instagram
"I don't have all the answers now, but I do know that the last few months with fevers, infection, and low potassium most likely took a toll on my sweet boys heart. I'm still in shock and going through the motions of dealing with all the details."
Bravely ending on a positive note, Michelle wrote, "I know Gray is playing cards with Gramma and playing Trouble with our beloved Diane. His dog Mo is right at his side as he never left him"
Grayson was diagnosed with brain cancer in 2006, just three days after he turned 6, according to Michelle's support website.
Smiley's fiancée and former RHOC star Gretchen Rossi also mourned the loss on Instagram, sharing a series of memorable images of Smiley and Grayson over the years.
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"Our rational minds told us this day might come, but our hearts always held onto the hope that this day would not come to fruition 💔," wrote the reality star, 44. "We are beyond devastated and heartbroken over the loss off Gray. 😔 Grayson Arroyo-Smiley was such a special human being."
Remembering him as "an angel on earth" and "an incredible warrior" she saluted Grayson as "a warrior like no one has ever seen before."
She also recalled how he managed to stay positive despite his health struggles.
"He fought every day through his pain and discomfort to find joy in the smallest of things. He was always singing & dancing & laughing despite his circumstances. He was a true beacon of light and hope."
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She praised Grayson for the difference he made "Although his life was cut short on earth ... "
"The Lord knew his life had purpose and would touch many along the way."
Also staying positive, she said she and her family are "forever grateful for the many lessons learned from this little man, and will miss his beautiful spirit immensely. May the Lord hold him in his hands, renew his broken body, and give him life again. 🙏🏻"
"This isn't goodbye but rather see you soon little man…..We love you Grayson. You will forever be in our hearts ❤️ Love your Dad, Gretchen & your sister Skylar Gray"
She ended by asking supporters to respect their privacy "as we mourn this unbearable loss"