I have booked business class flights for my trip, if I had to return to Australia during my trip because my son is hospitalised, will my return flight be in business class also?

You should refer to Benefit 5 Trip Cancellation Expenses and Benefit 6 Trip Disruption Expenses to see whether cover is available under the policy.

If your situation is covered under your policy, you can have cover for the same fare class that you had originally booked. This means that if you had booked return business class tickets, then you would likely have cover to return to Australia on business class tickets if your son is unexpectedly hospitalised.
As per Exclusion 2 in the PDS, In this situation, you are required to do everything you can reasonably do to reduce your loss as much as possible. Therefore, we would expect that you would contact your airline to change the return date of that original business class ticket, and submit a claim for any out of pocket expenses that you incur as a result of the change.
You are free to make a claim, and the Claims team will assess your claim and make a decision based on your particular circumstances. 

Cover is subject to the policy terms, conditions, limits and exclusions in the Product Disclosure Statement (PDS).