My 11WO DS does much the same kind of thing.... drenches a BBB in a couple of hours, and leaks out of most AIOs I've tried in one big wee. It's always a tradeoff for us whether we construct a "colossus" nappy that will keep him dry but is so big that his little legs stick out sideways, or whether we keep the nappes trim but change outfits at every nappy change.
We have had the best balance (between trimness and absorbency) so far with - gasp - basic cotton terry flats in PUL covers or wool shorties, and just changing him every time he wees. Sometimes a bamboo hourglass or some trifold booster tucked in the terry gives us a little longer too. I keep buying cute AIOs (TIC, bubblebubs, KIC, magicalls etc). but I keep going back to kite-folded terry squares.
These days he wakes at night not always cos he's hungry but frequently because he's wet through everything, so we've been trying a series of "colossus" nappies to see what gets us the longest sleep. Bubba J hemps are looking good, so are Sandman night nappies, both boosted right up (inside & outside) and wrapped in PUL then wool. We have had to tilt the basinette matress up a bit, because his nappy was so big that he was lying with his head downhill, which he didn't like very much. Poor little guy - stuck like a turtle on his back. But relatively dry at least.
