Bexley set 4 NFER Nelson multiple choice papers- english, maths, verbal and non- verbal reasoning. In order to get a selective place you need to get a high enough aggregate score(changes yearly based on scores achieved). The highest 180 scores are guaranteed a grammar school place, the other selective scores are then subjected to the school's admissions criteria such as medical needs, siblings, distance etc.
I would suggest buying practice papers from either WHSmith or similar, and going through them with your son, your Yr 4 son may well enjoy some of them too! Once he is used to the format, start building up speed etc. The same format and types of questions seems to be standard from year to year, so this should make him familiar with what he will come across on the day.
10 passed from my daughter's class this year and as far as I know only 2 had tutoring rather than just doing papers at home.