ARRRRRRRRRRGHHHHHHHHHHHHHHHH!!!!!!!!!!!

Hope you find something that works for you soon. I agree with Hanna about the cotton outers. I have one HL fitted with a cotton outer and the rise is way lower than my BV ones.

Have you tried folding up a flat nappy into a pad and putting it where he needs a little more absorbancy? It would add a little bulk but hopefully not too much.
Also have you tried soaking the nappies in a bit of vinegar (with cold water rinse) before washing? It gets rid of the urine/faeces which bonds to the soap residue. You need to use quite a lot of vinegar the first time you do it, I used about half a bottle of distilled white vinegar.