When to switch to bigger prey?

So for every snake I've gotten, its been an adult and eating the right sized prey for the rest of its life. However I am now raising, from 7 weeks old, a baby red tailed boa. He's 18 weeks now and I'm giving him 2-3 frozen thawed fuzzie rats a week, but I was wondering...when is he going to be big enough to switch him to bigger prey? Or how should I know when to switch him over? He's real skinny as of now, and around 1.5 - 2 feet long. Here's a video of him I made just a few days ago. a href="http://www.youtube.com/watch?v=-618CCAiRbY" rel="nofollow"http://www.youtube.com/watch?v=-618CCAiR…/a Any idea's when to switch him to bigger prey? Thanks so much!