Is in Associated Board? If you go to their website, there are recordings to download of all the grade pieces (and accompaniments). There are CDs available as well if you are near a music shop. They don't have them for all instruments, but violin is one they do.
If it's another board, or just a book of graded pieces but not for an exam, then it might be harder - there are various online sites that would teach you how to read music, or virtual keyboards that you can play if you can work out what the notes on the staff are to hear the intervals (you'd have to know the rhythm too to actually play the piece though).
Or more realistically, let his teacher show him and you can just listen and see if it sounds vaguely tune-like!! If anything is really wrong, you might just be able to hear it.
Most beginner violin books have pictures of the four strings and the first few fingers on each one, and which notes they are, which won't help you know what it should sound like, but will show approx where the fingers will go at least, and then you might be able to hear if it sounds in tune.
You tube also has a lot of graded violin pieces if it's some of the main exam board pieces.