I'd get it on RATs right away. Start with offering rat pups. If you're not going the rat route, just offer appropriate size mouse every 5-7 days.

I start all my babies on live rat pups. It peaks their interest alot. They easily switch to FT 5-10 meals later.

Basically, just keep trying to feed it every so often, until you notice it starts dropping down in weight, then you may have to assist feed(fairly simple once you know what/how to do)