"Freezing" or "rigid" fish are usually in osmotic shock, if this condition arises quickly, like within hours, it's usually due to changes in PH or SG too rapid. It's best to alter only 1 parameter at a time and slowly, as then if osmotic shock happens - you know pretty much what parameter caused it. The best way to releive this condition is to know which parameter is causing him to act "rigid" and reverse that parameter slightly - then wait and see if he recovers.
Sadly, osmotic shock is usually not something the fish will survive, as eventually the shock affects respiration, which slows to a stop. But in minor cases of it the fish can return to normal. Then decide on a slower change to his water parametes.
