Jett, who had autism and suffered other serious health issues, died at the age of 16, after hitting his head on a bathtub when he suffered a seizure during a family trip to the Bahamas.
Travolta shared some fan art of his son, and wrote alongside the portrait: "A painting a fan made of my son Jett! Happy birthday my son I love you!".
"Happy Birthday Jetty, my sweet love," Preston captioned a slideshow of photographs that included images of her and her husband with their son.
The post comes a week after the 56-year-old "Sky High" actress shared a black and white photograph of her and Travolta kissing and cuddling Jett, in honour of Autism Awareness Month.
The couple, who wed in 1991, is also parents to daughter Ella, 19, and son Benjamin, 8.
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