The "American Crime Story" actor and his wife took to Instagram on Saturday to pay tribute to their eldest child, who died in 2009.
Jett, who had autism and suffered from other serious health issues, sadly died in 2009 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 off some fan art of his son, and sweetly 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 photos that included snaps 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 photo of her and Travolta kissing and cuddling Jett, in honour of Autism Awareness Month.
"To my sweet love, Jett... you are in our hearts forever," she wrote. "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 #autismawareness #autism."
The couple – who wed in 1991 – is also parents to daughter Ella, 19, and son Benjamin, eight.
Speaking in 2012, Travolta confessed that he contemplated retiring after death of Jett, saying, "I decided it was OK to go back to work. Because I had even thought about retiring at one point, because it just felt like too much."