Take him out of the tank and put into a plastic container of fresh dechlorinated water. Change his container completely each day with dechlorinated water. Depending on your room temperature, if above 22deg celcius, may be necessary to put him in the fridge (as long as it isn't lower than 5degrees celcius).
When putting in fridge, make sure:
You cover his container with a lid, and wrap in a towel/tea towel/dish cloth to stop the temperature spikes and light when the door opens.
You keep a bottle of dechlorinated water in the fridge beside his container - this is so you can do complete daily water changes each day.
Offer food at waterchange times - but he may not necessarily eat, as metabolism slows down in the fridge.
Regarding your tank:
Have u tested the water for ammonia, nitrites and nitrates?
If u don't have a test kit, get the water tested at your local petshop/aquarium;
as it may be a water quality issue (ie: high ammonia/nitrite levels)