If he is in set 3/4 do you know exactly what this means? So, sometimes in bigger secondary schools, they split the year in 2 so there are 2 set 1s, 2 set 2s, 2 set 3s and 4s.
This means if he is in set 3 he may be (well) below the average of the year. He may well not be covering the same difficulty of work as the higher sets and if he is in a class with poor discipline and /or he is not trying himself he will be missing a lot of the foundation work for GCSEs. This is particularly the case for subjects where you are building on a previous year's work eg Maths, Modern Languages.
You quote what seem to be high marks, but if these are in a low set, it may be lulling you in a false sense of security. 60% in set 3 is not at all the same as 60% in set 1 as the work may be considerably easier.
Maybe you need to supervise his homework ie be physically in the same room as him, check what exactly he has to do/learn, go through it with him and test him on it until he has mastered it. ( You will learn lot too! ) It is important that he moves up sets before GCSE. If he isn't doing homework/ paying attention in class / revising, he will fall further and further behind.
If he hasn't any homework to do ( this will be his first ploy) then you ask him to get out the books for the different classes he has had that day and go through the class work with him and then test him. It may be in a text book or just what he has written in his exercise book. My daughter and I used to do 2-3 hours a night minimum.( It can be presented as you asking him to 'explain it' to you rather than you telling him to learn it.) You have to be cheery and interested for it to work.
I studied 3 languages so you can imagine the amount of rote learning involved.
You should not just be leaving it up to him. So, eg, if he has a list of 20 words or phrases to learn you go through it with him, noting the genders, the tricky bits of spelling, then cover up the English and ask him to tell you the meaning of the French words. Repeat until he's perfect (orally).
Then, the more difficult bit, cover up the French and ask him to remember it when looking at the English (again orally). This may take 5 or more times.
Finally do the same but ask him to write down first the English meaning when he looks at the French word, then ( more difficult) write the French words when you provide the English.
When he can do this with no mistakes he's finished.
A similar approach to learning any subject is necessary so eg, breaking it down into small sections and trying to write it down from memory but you will need to start off doing it with him until is second nature for him to learn it.