It is not that, he knows that 1Km=1000m and 1m=100 cm, the issue is when you want to transform Km in cm or reverse, I assume he memorized how to do it in school

It is not that, he knows that 1Km=1000m and 1m=100 cm, the issue is when you want to transform Km in cm or reverse, I assume he memorized how to do it in school but it was not natural, did not understand it and at home it applied the things he learned but in reverse. What surprised me was that he could not realize how absurd the results are. A thing I learned by doing the homeworks with my son is that is also easy to think that the child learned/understood your nice explanations/graphics/examples he does all the exercises/problems correct and then 1 week later he won't be able to do the exercises again, so I will repeat them again after 1 or 2 weeks to make sure he still knows how to do it. This may be different with other children or different teachers