We had an OT eval for much the same thing, and we were given a bunch of core strengthening exercises. You might be on the right track given the other things you describe.
The recommendation didn't really make sense to us - he can swim 50 yds of butterfly, but has insufficient core strength? DS doesn't really tire that easily, either. However, DS would also describe the feeling of being tired when writing. (We since have a dx of dyslexia and dysgraphia, so the tired feeling in our case may be due to the mental effort required in writing.)
We got the biggest bang for our intervention buck by making certain that the seating arrangement was at the perfect height and the feet were comfortable.
