Yeah, prefolds are tough in terms of keeping poop off the cover. It may not be true for every user all the time, but generally speaking poop on the cover is pretty normal for using prefolds. That said, different folds can help a lot so I'd second the suggestion to experiment a bit.
From the sounds of your first post, you're using pull-on covers, is that right? You mention it's messy getting the cover off without spreading poop on his legs so that was my first thought... Anyway, if that's right, maybe try a velcro or snap cover. It wouldn't help in terms of keeping poop inside the diaper, but at least it would make changing a little less messy.
Also, the juice may not helping in terms of stool consistancy (juice is known to cause loose stools). You could try maybe watering that down (say half/half), reduce the amount or even cut it out completly if you can. Sub with water, breastmilk (either at the source or in a cup), formula or a small amount of whole milk. Just a thought...