ruling out any serious medical condition, i would say your son needs some SERIOUS probiotics. and the ones you are using now are almost certainly worthless, just like 98% of all probiotics on the market today, trust me i spent a fortune on probiotics before i found the two that work. i would advise you to use LB17 but i do not believe that it is available in australia. too bad since its simply the best probiotic on the planet.
however there is another really great product i would suggest that does ship to australia. the website is here
the product this guy makes is far and above anything on the market save LB17. VERY pure and VERY strong strains of bacteria. i would suggest you call him and ask which blend he might recommend. off hand i would say get the Custom probiotic blend of six cultures, 260 billion cfu per gram. It consists of L. Acidophilus, L. Salivarius, L. Plantarum, L. Rhamnosus, B. Lactis and B. Bifidum. But maybe another strain would be better for the child, i cant say for sure.
you can then get a yogurt maker and use this as your starter culture. use plenty of the culture to make your yogurt with. make it nice and strong. then add your child's favorite flavoring to the yogurt. this will do wonders. trust me when i tell you that this simply doens't compare to taking some lame, old, dead probiotic from a bottle. completely different experience.
if you want to stay away from dairy then use this to ferment vegetables with, although you might have a hard time getting him to eat fermented veggies. you can perhaps try to ferment carrot or apple juice, that might work.
if you fill him with enough strong bacteria he will eventually go. dont be afraid to make it strong and give him plenty of it.
of course i cant guarantee you that this will work but this approach did solve long standing constipation problems for me. good luck with it, i hope things improve for you.