The system is explained here - hope you've got your brain securely belted in for the ride!
The Bucks pass mark is 121 (I am told that, in theory it can vary, but it has been 121 for as long as I've known about it) and the top mark is 141.
As your son is in the youngest quarter of the year he is at a slight advantage, as you will see from the tables. There is an unproven belief among Bucks parents that the system favours the very oldest children (Sept - Nov birthdays) because they are more mature, and the very youngest (June - Aug birthdays) because of the standardisation process. Mind you, every time I say that, someone appears to say quite the opposite!
To achieve 121 your child will need - eventually - to be gaining a "raw score" before standardisation of around 86% in practice papers, but you have a long way to go before you get to that stage.