My DC has hypotonia (couldn't roll over or sit until very late, didn't walk until after 18 months). Everyone including the pediatrician for the longest time acted likely it was no big deal, so we didn't even recognize that we should address it until about age 5. Before that, we had done gymnastics and swimming with minimal improvement.
The two things that helped the most were:
This online book I found, with activities you can do at home.
http://www.ot-mom-learning-activities.com
We turned these activities into a game with both kids, and would try to do some every night (or every other night). The kids had fun (and it is better than watching TV) and my DC core strength really started to improve.
About 6 months later, we started TKD. That has done wonders for his core muscles, as well as his confidence, and concentration.