Have u tried doing step-by-step?
For example, after establishing that there are 9 dogs in the kernel, explicitly test his understanding of the phrase "6 leave". If he do not understand immediately, resort to using physical objects or drawing of simple diagrams (not models yet but perhaps 9 dogs’ faces and circle 6 of them). After he understand the physical meaning he can move on the procedural meaning of 9-6.
Then if he understand the first step can move to "4 come in".
However judging from what you said, i presume he can probably solve a 1 step problem sum. In that case teach him to see step-by-step by covering up the "4 come in" part.
Its important to him to be able to learn how to see step-by-step by following the english phrases in sequence.
Please comment whether this works for him.