I failed English at High School level but am now about to complete a Master's degree anyway (also have a pilot's licence). HSC does not, and should not matter.
It is a waste of time, energy and money, trying to beat the system; especially dealing with the Corporation Against Sensible Aviation. It is much easier to undertake something like the Tertiary Enabling Program (TEP) and get the required "pass marks".
Should be a walk in the park for someone as bright as your young bloke. If TEP gets the required passes for entry into uni programs for people who did not complete the HSC, it should be good enough for the bureaucrats.
Good luck with it all (especially dealing with CASA).