Jackson has been holidaying in England as part of his gap year.
Earlier this year, Cassandra Thorburn did a tell-all interview with the Australian Women's Weekly claiming Karl's friends and family had "discarded and disposed of" their children in the wake of her split from the TV star.
Following Cassandra's comments, Jenny posted an Instagram birthday tribute to her grandchild, Jackson, as he turned 19.Alongside a picture of a birthday card, Jenny wrote: 'Happy 19th birthday Jackson.l hope you have a great day celebrating while overseas.I really hope the universe is able to get you see my/our wishes.
"Love you heaps Jackson Little nanny 😘🎂"
The teenager is yet to respond to his grandmother.