What is the correct phrase for the relationship between me and my son's father/mother in law?
It's in your question.
There is no special term in English for a person's child's parents-in-law.
They are your son's parents-in-law.
Now, this is not to say that English wouldn't be enriched to have such a term, but it doesn't and there we are. It also doesn't have terms to distinguish between the flavors of aunts and uncles, and a few others that do exist in other languages.
I hadn't realized when we had daughters, that I would also be gifted with sons when they married. My affection for their parents is beyond the tepid description "my son-in-laws parents". "Affinity sister/brother" is a much better term, as connected by choice albeit our children's choice. so...this holiday season I plan to introduce Kay as "my affinity sister who is Dave's (my son-in-law's) mother".