Hey Mum of triplets.
I'm a aussie too, I found the flutter to be quite good, once you get the technique right.
I aslo found the pep to be pretty good as well, I used both of them alternativeliy..
When I was doing PT treatments I would use pep, flutter and percussion, different days different things helped..
Ask the physio department, they usually have them to try first. that way your son can have a go at it, and see how he goes before outlaying the money to buy one.
All Up I have a pari pep, and astro pep, and a flutter, the pari pep gives you the choice of doing pep by itself, or you can use it in conjunction with you pari ned and do your treatments while doing pep as well, I actually found that by doing the pep during my treatments assisted in getting the ventolin and antibiotics into harder to reach parts of my lungs, cause you change the way you breath...
But get in touch with the hospital and see if you can get one of them try, or see if you can try all three, some ppl prefer different ones, and some have different results.
I would recommend all three, but thats just me, I had a lot of junk to get up, and the variation of pressure from pep, and vibration from the flutter etc aided in getting the stuff up.
Good luck