I agree with you on how the middle 50% numbers work and I think that means we agree that if accepted your son would be in the bottom quartile in BOTH Rank and SAT.
That is, he is not a lopsided student with a strong rank and low score or vice versa. He is just a plain old bottom quartile student for this school. (I don’t mean to be insulting - I’m in a similar boat at many schools that I had thought of applying to.)
It’s a relatively long shot to get admitted to a large public school in this situation without other strong factors in his favor. Transcript and scores are the primary factors most schools consider and neither of those is in your son’s favor.
Whether the bottom quartile in Rank and SAT includes some, many or no accepted applicants with Rank or SAT below your son’s is unknown and generally unknowable.
Whether there are some, many or no accepted applicants who are BOTH in the bottom quartile for Rank AND the bottom quartile for SAT is also unknown and generally unknowable.
And, I’m sorry but I don’t understand the 40-50% comment you made. Are you trying to figure out if your son’s stats put him towards the middle of these bottom quartiles? If so, I don’t think that’s possible. Unknown and generally unknowable.