(What is JHK’s first language?)
I guess, there’s one French as a Second Language teaching method you could apply to English.
It has to do with body language: you associate one gesture with each preposition. Like:
Behind: you point your thumb back and forth over your shoulder.
Between: you join your 2 hands and move them up and down.
Over: you move your hand to and fro over your head…
…and so on.
(I struggle to make myself clear in English here , sorry X Mode!!) Hope you can still get the idea.
You teach JHK the prepositions and their “body sign” at the same time, it’ll help him/her to better conceptualize their meaning. Then, each time he speaks and omits a preposition, use its associate body sign and he should understand his mistake right away…
Have you already tried this method? If not and if you ever use it, I hope you’ll tell if it worked.