Some fish are more prone to ammo poisoning than others. But just because he's not showing signs doesn't mean he's not suffering. Ammonia burns generally show up after the fish has been healing it's wounds. He could still be having his gills and skin burned by the chemicals and you cannot see it happening outwardly. This will open him up for infection and is probably very uncomfortable if not painful for him. It lowers his immune system and other natural defenses (like his slime coat) to have to fight the effects of the ammo.
Just do 25%-50% water changes depending on severity of the amount of ammo in the water to get it in check.