Which bit of the aurals does he find hard? You can practise the clapping the beat to the radio in the car - I do this all the time with any kids in my car - (my kids' friends must think I am total weirdo) - just get them to clap the beat and then move on to emphasising the 1st beat, and then working out time sig.
Singing echoes - first step is to sing one note back from the piano, to check they can pitch that. You can practise clapping echoes back, because rhythm is part of the echo, it's not just pitch. Try singing echoes to him to sing back, this is often easier than pitching from the piano.
Difference - remember it's not just 'pitch or rhythm', it's 'what is the change' so practising describing what has happened (in terms of 'there was a lower note towards the end, so it was a pitch change) is useful.
Musical features - again you can do this in the car - listen to the radio, and think 'is it loud or quiet, is it smooth or detached, major or minor, fast or slow'. Just raising an awareness of how to listen is good.
I wouldn't delay an exam if aural was the only weak area though. I remember Clara Taylor (previous Chief Examiner) saying 'if you're no good at aural, just have a go, don't worry about it, it's just one section'. And if you got everything completely and utterly wrong, but gave an answer you get minimum 6 marks anyway! But is much more likely to come out on the 11/12 borderline, as he'd be unlucky to get everything wrong!