Once he starts he will get texts books for most subjects so you can then get an idea of what is going on.
The curriculum will change by greater or lesser amounts on a regular basis so the experience of what was done a few years ago will not necessarily be true for the new intake.
I wouldn't try to prepare him specifically as he will more likely get bored when term starts!
In Maths it is almost certainly a good idea to keep things ticking over - but with the harder year 6 stuff rather than pushing forward too much - and possibly with French.
If he already done some then keep that ticking over or if he hasn't done any then maybe just a bit of fun oral work (masses of web sites) so he isn't totally phased by it. Most of the boys wont have done formal French in the way it is done at secondary school so no need to worry but if he isn't familiar with speaking at all then he might be happier if has tried a few words beforehand.
When he does start in September the school encourages parents to keep track of what is going on especially by keeping an eye on homework. Obviously there is no point in doing it for them! but you can check it looks presentable and reasonable effort has been made etc. If there are problems then staff prefer the boys to ask for help straight away rather than waiting until homework is due in and most will respond to emails from the boys if they need help. Sometimes you can help by reading through a text book with them and talking it through to help them figure it our for themselves. Many boys will still need guidance for homework that requires research.
The school also uses Moodle and some homeworks will be detailed on that.
They are pretty good at settling the boys in at the start of term so no need to worry - but do encourage DS to write details of homework in his Diary and make sure that he understands what is needed before he leaves the classroom
Hope some of this helps.