Regardless of "relation" he owes you one heck of an apology. If not, build him a paper airplane. he may be your father in law but its your son. You make the rules regarding him and you need to back it up. Don't allow him to go flying with you and your son. It's your quality time. Don't be sucked into the bad day he's having or he's just "old."
Your wife should clearly understand this.

Let me give you and example. I'm 52, I spent 20, retired 13 years ago. I always kept in contact with family by phone with visitng every few years. Well in 1997, if had been nearly 10 since I had made it back, My boy was 12 and I forgot what a "Mouth" my dad had.

2 days into a week long visit. I finally said enough around the kid. I mean I swear but it's not part of every word from my mouth. There was no long argument over it. He just gave me 'the look,' and i gave a better one back. It was actually funny for the remaining days as I would notice him catching himself as he was halfway into a real good one.