It's certainly on the tight side given the international arrival and the bus to the domestic terminal, but it should be fine 90% of the time. Or 80%. Or 95%, I don't know the statistics, but if VA reckons they're happy to take the risk then I would be too. I think as long as the incoming flight isn't late and he doesn't get lost it'll be fine.
The worst thing that can happen is that he misses the flight and has to spend a night in a Brisbane airport hotel at VA's expense before flying up on the first am flight. If that's not a disaster (ie if your son is old enough to take care of himself and you don't have to catch a ship in the morning or something) then I'd just accept that's a few-percent probability.