Definately try for a slimmer crotch nappy.
Gigi might pop in and reel off the rec'd brands! lol

But I have a similar issue with Charlotte in the she is quite skinny, and very long, so I need the slim crotch nappies and trimmer fit to give her the range of movement she need.
Sposies will give them different movement because they are thiner and cut differently than most MCN.

MCN (in my experience) are very individual for each bub - some nappies one mother will love and another they wont work so well.

The other thing is are the BBH OneSize nappies? Try to get sized nappies that fit your bub rather than going for a o/s as they tend to be bulkier, especially on a smaller bub.
The itti bittis will be good as they are low rise, so they should allow him to sit up (bend at the waist) more easily. Also the TotsBots i have a good for DD too. You could also try a pocket or a fitted and cover system that will allow you to manipulate the absorbency (bulk) of the nappy depending on the activity (home, play, sleep, car, shopping, etc)

Good luck - and dont give up on the MCN!!! lol