geo, doing butterfly and weak core strength are not mutually exclusive... a lot of issues with core can be coordination, balance and/or endurance. Swimming is a good exercise but does not cover all areas of core strength at all (not even butterfly). It means he can do well in what it takes for butterfly - but still have weak core in other areas - including sitting to write.
From the session I watched with DS's OT, it is not one set of exercises but a wide range of seemingly unrelated excersises to cover the many areas that falls under the general "core strength".
