I wouldn’t put anything in his backpack. I’d arrange to have it sent to him closer to the time. Get ideas from him after he arrives. When I was backpacking my family sent things to post restante, it’s a service travellers can use to collect parcels. Australian post offer the service but can’t remember its name now.
Its far easier to buy bits of home these days but when I was travelling ( over 20 years ago) I missed little things I couldn’t buy in NZ & OZ like drifters, pickled onion monster munch, cherry coke, lucazade... There are a lot more Brit shops to buy these things now and more uk stuff is stocked in every day shops.
Also think about where he will be over Christmas, if he’s in a Sydney he will have so much at his disposal, if he’s in the outback he won’t.
I know you don’t want to hear this, but if he’s only going in Oct he’s not going to be home sick by Christmas. Australia is bustling, he’s coming into summer, lots of public holidays and events everywhere. He’s going to be having the time of his life.
I’d definitely subtly find out if there is anything he can’t get, his favourite crisps etc.. I couldn’t stand crisps in nz & oz. One thing English people loved getting was things like English marmite, there is a huge competition of which is better English or Australian/NZ my English friends didn’t like it over there and kiwi/ozzy friends don’t like the English version. There is a whole chocolate thing too, Cadburys choc tastes different over there.
For my DH the thing he missed at Christmas was terrys dark chocolate oranges.