Sorry to hear about this situation, and "welcome" if you are new!
For a comment to be made along the lines of "there is a historical problem" is not exactly helpful. I'm presuming what is meant is there is something that you have said or done in the past that he has taken offence at.
So, without knowing what you are supposed have done, what does he expect you to do? Play 20 questions until you get the right answer? Fall at his feet and grovel for every time that you might have got something wrong? "Oh Son, I'm so sorry that I didn't let you have an ice cream on that day trip to Bournemouth"?
It seems this situation cannot be resolved until somebody actually says what caused it in the first place! So, your DIL knows, her parents know, and of course your son knows.
Shame that he feels sulking is the answer, he needs to sort things out.
Life is too short for this sort of behaviour!