Im a father of 5. I went OTD at age 29. We still live together 5 years later. I can’t say it was always easy but things got better with time. I think the key is respect. Simple respect to another human being. If there is a mutual respect everything can and will work.
However, If you’ll approach this, like some commenters here suggest, like he has a “problem” i can guarantee you things will go down the drain very very fast.
Its ridiculous how people immediately assume either a GYE issue or mental disorder etc. I don’t blame them because thats what the rabbis say. Truth is, some people, especially those who drift away later in life, become OTD for intellectual reasons and after a long painful journey. Yes internet may be what helped us reach our conclusions, but we are perfectly healthy in all matters.
In such cases the only thing that may work is ‘convincing arguments’. Everything else is a waste of time and energy.
Good luck.