Yes, it is a bit late - and you don't want to panic him just before the test.
Sight-reading: I always say it is more important for the FINGERS to learn to recognise where they should be, rather than thinking of the NAME of a note, and then working out how to play it. Probably it is a good idea to practice on a wide range of music, of different genres; find stuff in charity shops or car-boots, and as long as it is of a similar difficulty, and they are notes he knows, it doesn't matter what style it is.
Musical periods: the only way to 'crack' it is to listen to a wide range of music - and these days there is a vast amount on YouTube and other sites; listen to Radio3, and even though the periods may not be featured, try to ELIMINATE periods that are not relevant, and get used to hearing ones that are 'possibilities'.
"ABRSM information 18 marks
To sing or play from memory a melody played twice by the examiner. The melody will be within the range of an octave, in a major or minor key with up to three sharps or flats. First the examiner will play the key-chord and the starting note and then count in two bars. (If the candidate chooses to play, the examiner will also name the key-chord and the starting note, as appropriate for the instrument.) If necessary, the examiner will play the melody again and allow a second attempt (although this will affect the assessment).
To sing six notes from score in free time. The candidate may choose to sing from treble or bass clef. The notes will be within the range of a fifth above and a fourth below the tonic, in a major key with up to two sharps or flats. The test will begin and end on the tonic and will not contain intervals greater than a third, except for the rising fourth from dominant to tonic. First the examiner will name and play the key-chord and the starting note. If necessary, the examiner will help the candidate by playing and identifying the correct note if any note is sung at the wrong pitch.
(i) To answer questions about two features of a piece played by the examiner. Before playing, the examiner will tell the candidate which two features the questions will be about. The first will be one of the following: dynamics, articulation, tempo, tonality, character; the second will be style and period.
(ii) To clap the rhythm of the notes in an extract from the same piece, and to identify whether it is in two time, three time or four time. The examiner will play the extract twice (unharmonized), after which the candidate should clap back the rhythm. The examiner will then ask whether the music is in two time, three time or four time. The candidate is not required to state the time signature.
Piano requirements and information: Aural tests
Listening lies at the heart of all good music-making. Developing aural awareness is fundamental to musical training because having a ‘musical ear’ impacts on all aspects of musicianship. Singing, both silently in the head and out loud, is one of the best ways to develop the ‘musical ear’. It connects the internal imagining of sound, the ‘inner ear’, with the external creation of it, without the necessity of mechanically having to ‘find the note’ on an instrument (important though that connection is). By integrating aural activities in imaginative ways in the lesson, preparation for the aural tests within an exam will be a natural extension of what is already an essential part of the learning experience.
In the exam
Aural tests are an integral part of all Practical graded exams. The tests are administered by the examiner from the piano. For any test that requires a sung response, pitch rather than vocal quality is the object. The examiner will be happy to adapt to the vocal range of the candidate, whose responses may be sung to any vowel (or consonant followed by a vowel), hummed or whistled (and at a different octave, if appropriate).
Assessment
A number of tests allow for a second attempt or for an additional playing by the examiner, if necessary. Also, where there is hesitation on the part of the candidate, the examiner will be ready to prompt, if necessary. In any such cases, this will affect the assessment. Marks are not awarded for each individual test nor deducted for mistakes but reflect the candidate’s overall response in this section."
Depending on how he does this time, you know what he needs to prepare for a future occasion. And at least it's not as life-altering as CGSEs and A levels might be.