Hmm. I have a pattern in progress in my head for this, as my next thing to do is to make cloth pullups for night time bedwetters. I picked up these good nights, that are actually paper boxers with a diaper on the inside, for our trip that we had to postpone. I think I like the pull ups better myself, but my son likes these. Do you want a wetness barrier?? Because if you don't, and you want him to feel wet during the day, then I would think that you can easily change your prefold into a prefitted, and then attach the front and back to boxer shorts instead of regular underwear. I don't see how it would work with regular boy underwear. There is a prefitted tutorial on DiaperSwappers that is SUPERB! It shows it all step by step in pics and instructions. You cut into the prefold on the sides, making what looks like a contour diaper, but don't cut down the middle. Take those flaps and sew them to the middle (or together in the middle), and then thread elastic through the legs. Too cute!! From there, you would just sew them into the boxers.
However, if you are wanting a wetness barrier, then I am planning to make prefitted of sorts, with knit on the sides so they can be pulled up and down. And, then having a fleece/PUL shorts or pants as the cover.
Actually, like underwear, you could do the same thing. Make the diaper into a prefitted, and then add the knit on the sides and elastic to the back and front to make it like an underwear with more padding to catch accidents. Is that what you are needing? Kymberli