First dinner with my husband's brothers, wives and parents was a shock. They didn't talk. Everyone ate with their forearms wrapped around their plates. And they shoveled.
I am an unapologetically sarcastic person.
Several years, many dinners, and numerous attempts to engage the family in dinner conversation later, I asked him later my husband if he was worried that I'd steal his dinner. I gently explained the merits of basic table manners. That was a complete miss. Who is this Emily Post to decide what constitutes acceptable manners?
I tried a few other tactics too, all failed.
He's one of those people who needs proof to understand, so on our next dinner out I imitated him to an extreme. The point was driven home after a few times.
Thirteen years later and I think he has better table manners than me, and I was taught by a master

He needs those manners — he's often dining with the upper echelon of his industry.
Make the effort to teach your son-in-law. Whether or not he learns anything is up to him.