Absolutely grief stricken and devastated - husband has been diagnosed with terminal cancer. How do we tell our 18 y/o son?

Our world has been shattered by the diagnosis of terminal cancer of my husband. We are older parents and our only child, our precious son, is just 18-years old. He is exceptionally close to his Dad who has been his primary caregiver throughout his young life. My husband and I did a role reversal during my pregnancy so I have worked all these years as the sole breadwinner, whilst my husband did an amazing job of caring for our son. The three of us are so close and we have to break the life altering news to our son this week. How do we tell him? And how do we live our lives without my life partner and my son’s Dad, who is his superhero? This has all happened so quickly and our lives have been devastated in just two short weeks. How do we cope? I am rapidly unravelling and cannot bear destroying our boy’s life and future. And my heart is aching for my husband and for the unjustness and cruelty of this catastrophic disease.