Your post reminds me of a writers group I belonged to in our tiny mountain town. We decided, as a fun project and an exercise in creativity, to write a novel together. We didn't collaborate, but instead, one member started a story all on her own, a surprisingly serious story of a kidnapped woman tossed into a dark basement. She passed on that first short chapter to a second member. At the next meeting, we learned what she did with the story, which was passed on to a third member. We never gave the next writer any direction. Each writer could take that story wherever they wanted.
When we first started our little writers group, we met once a month. After a few months of this project, we were so excited to see what would happen in the next installment, we changed to every other week! We worked this story for months. In the very beginning, the piece was limited to one page. Quickly, we tossed that requirement and just let the writer write, however many pages it went.
The twists and turns the story took were dramatic. Once, for my chapter, I continued the vein of the story and wrote that our female sheriff's deputy, despite being poisoned herself, saved the lives of three women. My chapter ended with the deputy deathly ill. The next installment he killed her off! I bawled. That was years ago and I'm still mad!
Our creativity blossomed and it taught us to pay attention to the previously facts of the story and how to write our own installments to keep the story going with a common sense and exciting narrative.
This could be a great joint and bonding project between you two and really work the creativity and to learn how to study and write a story arc. Perhaps he can kick off the first chapter, with you taking the second.