I'd echo taking as many little gifts that are new (have you got time to go to pound shop or somewhere similar to stock up on cheap stuff that can be chucked).
Stuff that worked well for us were tonnes & tionnes of stickers, small books, colour stuff - the colour wonder books are great, small cars, all placed in party bags & brought out every hour or so. Let him chose some magazines at the airport & maybe a small gift. Playdough or plasticine would be good but not 100% sure if you can take them onboard now so you could check! Dvd player or laptop that plays dvds is great but battery power is short so maybe just use this as emergency bribery & try to recharge it in Singapore for the next leg of the journey. (dvd player also handy if you so out for meals in Perth to keep him occupied) Also keep a stash of new stuff for the Singapore-Perth leg. If he has a favourite toy, let him take that too.
Make sure you pack some calpol sachets in hand luggage in case he's unwell, change of clothes for him & at least a change of top for you & DH (just incase he's sick) & a cup with a straw so he can have a drink on take off & landing to help his ears (or let him have sweets if he has them!).
Take plenty snacks (raisins, breadsticks, fruitflakes, fruit, white choc, crisps & things he maybe doesn't ususally have as bribery!) Don't rely on the airline child meal cos not always too pleasant & at least if he doesn't like it, you know he'll still have something to eat.
If you'll be doing any of the journey when it's his bedtime & you want him to sleep, try to get him changed into pj's so he knows it's sleep time.
Do you have a trunki that you can pack his stuff in? They're great for holding their stuff but also on the layover if he's tired, you'll be able to wheel him through the terminal on it & if he's not tired he'll be able to zoom around on it!
I'm sure he'll bee fine & often the kids are better behaved than some of the adults & just remember it is only 17 hours & you'll never see the people around you again.
Oh, & have fun!