Under "Search," for age you should indicate "adult" (for the mother) plus "teen" and/or "child" (for the son), depending if you want the boy character to be a teenager or a preteen child, or both if it doesn't matter to you.
Also, for spanker indicate "female" and for spankee "male."
These search parameters won't guarantee that all stories shown are mother-son, but they should narrow down the number of stories you'd be looking through.
I suppose you could also include the word "mother" or "son" in the synopsis search parameter...

--C.K.
P.S.: I recently wrote a lengthy comment on Adam Kessler's mother-spanking-son narrative, "The Three Bs," so offhand I'd recommend it to you.