The school is not allowed, under the Admissions Code, to say outright, 'You have a place' at this stage. All you have done so far is ever your son for a test to see whether he meets the school's academic standard (which he has
); you haven't applied for a place until you have submitted your CAF, naming Marling as one of your desired schools.
However, as the school admits 150 boys into year 7 and your son's rank is 150th or better and
at least a few of the boys who were also in that 150 will choose to go elsewhere, you can be more or less certain of a place for September. However, you should also name at least one non-selective school for which he meets the criteria as well (otherwise, in the unlikely event of not getting Marling, you will be allocated the nearest school with places, which may not be one you want under any circumstances).