The option you suggested - export the project and have him import it into his Local and continue development. Then do the reverse when you’re ready to hop back on the project.
If you’re using Flywheel for hosting, you can add him as a collaborator to the site so you both can push & pull from Flywheel. NOTE: I haven’t personally done this before so I’m not sure about best practices for version control aside from good communication.
Regardless of the route you take, I would make a backup for yourself just in case:
I hope that helps, let me know if you have any more questions!