Dorsey, 37, posted a throwback Instagram photo of a smiling Rivera with the couple's son Josey, now 5, at a restaurant about to dive into a birthday dessert.
"We can't say the word happy, but we'll say thank you for being a mother & giving me this sweet amazing boy," Dorsey wrote in the caption. His four-year marriage to Rivera ended in 2018.
The tribute hit home with fans and "Glee" co-stars still mourning the loss of Rivera, who died at age 33 after she disappeared during a July 8 boat outing with Josey on Lake Piru in Ventura County, California. Rivera's body was found July 13, with the cause of death ruled as an accidental drowning. She was laid to rest on July 24.
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"Glee" star Jenna Ushkowitz responded with several heart emojis. Heather Morris, who played love interest and fellow cheerleader Brittany to Rivera's Santana on "Glee," wrote in the photo caption, "This broke me. I love you."
On Jan. 12, Dorsey wished Rivera a happy 34th birthday on Instagram, posting a black-and-white photo of the family in happier days.
"Just as surreal as it is real that you're gone. If that makes sense, but none of this still makes any sense," he wrote in the caption.
On Sunday, Dorsey also paid respect to his own mother, with a photo of her holding Josey.
"Happy Mother's Day Grammy," he wrote next to the photo. "Thanks for all the support, unconditional love, all the help, your patience, thoughtfulness, and everything you do."
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This article originally appeared on USA TODAY: Naya Rivera: Ryan Dorsey honors first Mother's Day since tragic death