Why is changing provider a last option? Loyalty used to count for something but these days most larger companies know that most customers will pay the lazy tax. Case in point, Optus have a fantastic offer for 10Gb a month for $12.50. Its on right now but your father can't have it and keep his number because he is an existing customer.
I would suggest two options.
1) Take the $150 plan from Optus if you're happy to lose your number.
2) Port to the Boost prepaid plan of $240 for 240Gb for a year. Or about twice the data your father is currently getting for half the price. Plus free nation calls and a lot of free international ones if that's an issue. You can buy the sale price today and activate it when you want (just put in your current number to ensure it comes along)
As its a year long prepaid there's actually never any need to know if your provider is Optus or not. There's no bills or the like. The other big benefit is you avoid Optus egregious $10 'excess' data fee. In your case. Going 1Mb over your plan means a 25% monthly cost increase and with youtube its easy to do many time over in a few minutes. Prepaid means no more unwanted fees.
Another benefit is you don't lose unwanted data. Use 5Gb one month and 30Gb the next. It just comes off the 240Gb total.