Have Mrs ArGee grab an extra pair of his slacks/jeans/chinos and have him sit on several (one thick one may be enough) towels for the ride to the ultrasound.
I’m not proud. I’d wear Depends in the same situation.
I do hope it’s nothing serious with your F-i-L.
He wouldn’t let anyone grab pants for him without a lot of screaming and hollering, and he wouldn’t sit on towels.
However, he made it through. He thinks it was a complete waste of time. Of course, he’s not the one who cleans up if he has an accident.