Diaper Saga

So, Gareth has been consistently leaking out of his diapers every night for about 2 weeks and regularly for about a month before that. We've tried Pampers Cruisers in size 5 and 6, Pampers Baby Dry in size 5 and 6, Huggies Overnites in size 6, Pull-ups and a pull-up over a diaper. He is over 37 inches and weighs 31 pounds. His torso always grows first so we need extra height in the diaper more than the extra room around. He holds his belly button for comfort (and can unzip pajamas to get to it), which means he can shove a diaper out of his way to get to his belly button comfortably. He is leaking out the side of the diapers (his hip and thigh are wet on the side usually). He sleeps in pretty much every position all over his crib every night, so I know that doesn't help. Our pediatrician recommended Diaper Doublers, inserts to increase absorbancy, however when I went to Babies R Us to get them they told me they've been discontinued. Maybe they'll come out in a new model or something, but they didn't know when that might happen. He eats dinner around 6 then has a snack (applesauce) right before bed (about 8), but doesn't drink a full glass of water or anything with the snack. He's not really ready for serious potty training (he's 25 months) and we have a new baby coming in about three months so I don't want to start something I can't be consistent with when the baby comes. It's getting old, the nightly changing of the diaper, pajamas and sheets. And when he leaks out an hour or two before he should wake up he usually can't fall back asleep. So he's been cranky and tired during this whole process. Looking for solutions...