Clover,
My personal opinion is "no". Here is my experience with telling the in-laws what's up:
Any time I've ever approached my in-laws for help with my AH (he cares deeply about how they view him), they have taken his side and enabled him and *I* have been made out to be the a-hole.
Now...a few weeks ago AH got so drunk he fell face-first into our koi pond and I had to call FIL for help in getting him out. They got to see for themselves first-hand what kind of state their son is in and there was no denying it or blaming it on me.
Your son is a big boy, so if his parents want to know what the problem is in the marriage, they should go to him. He can either man up and admit his problem, or he can weasel around it and make it out to be your fault. He will eventually give himself away, though.