Young keelbacks and other colubrids are typically feed feeder fish. Easiest source of these is putting a small bait pot in a local creek and feeding the fish you collect. You can also source fish from pet shops, however you need to be careful of any potential chemicals they have been exposed to. Also note that a thiamine deficiency can arise if you solely feed goldfish. Frozen white bait can also be utilised. Once your keelback is large enough it can then be transferred over to rodents via scenting the rodent with marine fish and slowly reducing the amount of scent until its taking unscented items.
I will also point out that it is illegal to collect and keep any snakes from the wild. If this is where you have obtained your keelback from, I highly recommend that you release it back where you found to avoid being prosecuted for poaching. Not saying you have, just a warning encase. If you have obtained your keelback from a legal source I hope you enjoy keeping it as they are an extremely interesting and fun species to keep, certainly wish I had more of them.
Cheers, Cameron.