Are you referring to the GWABR or the GVBR? I suspect you mean the latter which does have a couple of substantial climbs, in which case you may want to switch topics, however ...
As a person who's built for the long flat runs, downhill and short or gentle climbs, I would urge you to do the ride this year. If you find the big hills too much to climb on the bike, hop-off and walk. You'll have plenty of company walking along with you and there's no shame in knowing your limitations.
In the interim, I'd suggest you and your son find some gentle hills to begin, and ride them over the next few months. And as they become easy, look for something a little steeper & longer. It's important you train to limber-up the muscles & tendons that will be driving you up hill.
Don't be too keen and ride big hills to begin, as that will only demoralise you ... start slow and low, then work up.
We look forward to seeing you in Vic in November.
Life is too short to bother with idiots