It’s probably the owner or occupant of the unit who’s responsible. If the strata scheme has adopted Fair Trading’s common property memorandum, it says lot owners are responsible for “fuses and fuse boards within the lot and serving only that lot”.
Even if the scheme hasn’t adopted the memorandum, Fair Trading says; “The supply of electricity is the owners corporation’s responsibility. However, any cabling within the unit’s internal walls that only services the individual lot, is the owner’s responsibility.”
That would suggest to me that the fuse box withing the unit is the owner’s responsibility.
Unless your son can show that the electrical supply to the fuse board caused the problem rather than, say, a fault or overload within the unit, then the strata manager is probably right.
Just as an aside, even when schemes haven’t adopted the Fair Trading memorandum, in the absence of any definitive by-laws or rules, Tribunals use it as a guide to who’s responsible .