We use Fuzzibunz at night at don't have a problem with leaks. Fuzzibunz have a microfleece liner which wick moisture away from his skin so he feels dry, and when you use hemp inserts, they're very absorbant. They're kind of pricey, but if you just get a few to use at night, the expense might not be too bad.
Or you could stick with the prefolds but use either a wool or fleece cover. Wool and fleece are both very absorbant. A lot of parents who cloth diaper come to really love wool covers. Fleece is a little cheaper and a little easier to care for, and almost as absorbant as wool. There are a lot of different brands of wool and fleece diaper covers to choose from, but we have used the Stacinator brand and been very happy with it. Again, they're expensive, but you could probably get away with just having one.