Why not just let him take the test, without special preparation?
The point of a placement test is to find out where a kid is now, and then place them into the appropriate class based on the knowledge they have. Prepping him for the test might lead to an inflated score, and placement into a a class he's not really prepared for yet.
Because standardized test taking is a skill. The 5th graders in the current system have been taught to that test: in the wording, style of answers, etc. Also the French system may not exactly align with the system the OP's son is moving into, but that doesn't mean he is not capable of working at the higher level.
In our district, the level of math one is tracked into in elementary school can have a significant impact on the math you take through high school. And it is not supposed to be true, but it is well known that your math level in 7th and 8th grade, affects your ability to test into the highly regarded local math&science magnet school. This may not be the case for the OP's new district, but it is in many.
Beth, mom to older DD (8/01) and younger DD (10/06) and always missing Leah (4/22 - 5/1/05)