|I think the premium sized prefolds would probably fit him for quite a bit longer so I'm thinking you could keep using them pretty much as long as you'd like, maybe even till he's potty trained depending on when that happens and how big he gets.
But if you're wanting to switch to the Sandy's I would work on trying to figure out why they're leaking. Premium prefolds are more absorbant but I would still expect that a small Sandy's would be plenty absorbant for a 6 week old, heck I'd expect even an extra-small Sandy's to be absorbant enough for a 6 week old.
Since you've done this before I'm assuming you've got the usual bases covered like not too long between changes, no fabric softener, making sure the diaper/tag is tucked in etc... By leak through do you mean that it's actually leaking like onto clothing and such or just that it's saturated very quickly?
Hmmm, I don't know, I guess he could just be a super peeer... Unless you really dislike the prefolds for another reason or you can find a quick/easy solution to the Sandy's, I'd just be tempted to not mess with a good thing. If he's a big enough boy you could switch to the large Sandy's in a few months and perhaps they'd work better since they're bigger/more absorbant.