Isn't that save based on received damage? Fluff wise if not directly...he will eventualy roll low or 1, but I think he has high enough fort that you should use other powers instead of waiting for that to happen.
SRD Entry said:Distraction (Ex)
Any living creature that begins its turn with a swarm in its space must succeed on a DC 11 Fortitude save or be nauseated for 1 round. The save DC is Constitution-based.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.