His school will be giving him homework and he will need to revise for end of unit tests or vocabulary tests or whatever.
That, of course, depends on the teacher. When he was that age, I was certainly "encouraging" my son to do revision for certain classes where there was no evidence of any homework being given. Teaching style and standard differs enormously between teachers. In year 9 he had a particularly lazy Physics teacher who literally hadn't given any homework at all during the first few weeks of the year. At half term, I persuaded DS to spend a few hours "revising" what they'd done in lessons and doing some practice questions - it brought it home to him that he could barely remember most of what they'd done, let alone know it well enough, so it acted as a bit of a wake up call for him to do something every week, even if nothing had formally been set.
My experience is that you can't rely on teachers to set enough homework - some don't set much because they don't want to spend the time marking it. Other set very "loose" homework such as "read the next chapter" which is very easy to forget or ignore. Teachers can sometimes also be the world's worst for lack of preparation and suddenly announce a progress test the next day, leading to panic revision that evening.
If the teacher seems to be giving regular, structured homework (and it's being marked), then fair enough, no need to do more in year 9, but if there's no sign of that, then I'd strongly suggest some encouragement to do revision for those subjects to make up for lack of formal homework. After all, if the teacher had given them homework, they'd have to find time to do it, so if they don't then it's perfectly reasonable to do revision instead.