"Happy birthday my beautiful Jetty," the actor wrote on Instagram on April 13. "I love you."
He also posted a black and white throwback photo of the duo. John's daughter, Ella Travolta, shared a snapshot of the siblings from their childhood days, as well.
"I love you Jetty," she captioned the image. "Happy Birthday."
Jett passed away at the age of 16 after suffering a seizure during a family getaway to Grand Bahama Island in 2009.
"We had a very full life, my son and I, 16 years," John said during a 2015 interview with "Entertainment Tonight."
"We did travel the world and he saw every place by the time he was that age. But, you know, you plan on a life that's a lot longer for your child and you always intend to go first. So, that's the difference. Regardless of the full life we did live together, you know, you wanted a full life for him."
While John and his wife Kelly Preston grieved in private, the couple would often pay tribute to their son on social media.
On World Autism Awareness Day in 2019, for instance, the actress, remembered her son, who had autism, by sharing a family photo.
"To my sweet love, Jett... you are in our hearts forever," Kelly, who passed away in 2020 after a battle with breast cancer, wrote at the time. "I send love to all of the beautiful autistic children and the wonderful people who love them. May we all shine and grant love and respect to children with special needs."
She also spoke about Jett's autism during a 2012 episode of "The Doctors."
"He was autistic," she said, "and he had seizures. And when he was very young, he had Kawasaki Syndrome."
In 2010, John and Kelly welcomed their son, Benjamin Travolta. And while Jett would forever be in the couple's hearts, the "Grease" alum described the birth of their youngest child as "nothing less than a miracle."
"For us it's been uplifting," he told "People," per ABC News. "He's given the house a renewed spirit and purpose. He's brought us a new beginning, and his presence has brought joy to all the people who have wanted the best for us."