Can a bad belly smack from a swim team diving block cause costochondritis in a pre-teen boy?

Diving: I assume you mean a starting block . That would be very unlikely as they aren't that high off the water . Anything is possible , but probably unlikely .

Possibly Yes: A hard blow to chest can cause an acute injury and resultant inflammation to the chest wall, giving rise to same symptoms as in costochondritis, (CC) which in mist cases doesn't have an apparent cause. Follow up visit with his doctor is advisable in light of his underlying Kawasaki's disease, although no special tests or treatment is needed in most cases of CC. Rest and OTC pain meds should work.