I'm no educated vet, but it sounds as if your fil and you have a close relationship. I do with mine. I email him each week just to tell him what's up with the kids and grands. How's the weather there, all that BS, but I keep in touch.
I have been tempted on several occasions to tell my H that his husband sat on my front porch swing and broke down and cried twice about how his son was acting, but that would be betraying a trust with fil. I suspect this is the case with your fil. He will always love his son, but he doesn't have to always like him or his choices.
