I would go with beet pulp and add a fat supplement like corn oil or rice bran. See if you can get your hands on some probiotics too. They are a digetsive aid that will make sure he's properly digesting and utilizing all of his feed properly. Keep feeding the oats, but just the oats (2-3 lbs dry weight, 1 kg).
Also, make sure he is thoroughly dewormed. I don't know what kind of worms you have to worry about in Australia, but I'd imagine it's the same as us in the USA, or similar. I would deworm him now with a double dose of Pyrantel paste (Strongid, Rotectin P) and Ivermective + praziquantel in 4 weeks (Zimectrin Gold, Equimax).