Take him out of the tank and put him in a container of dechlorinated water enough to cover him but so his feet rest on the bottom and he doesn't float. Put a lid on the container and wrap with a teatowel and put him in the fridge. Also put a bottle, or another container with dechlorinated water in it and also put in fridge at same time. This will be used to when you do the 100% daily waterchanges.
if unable to put in the fridge then as long as he is in a clean container/bucket of dechlorinated water that is below 18 degrees in temperature. Change this 100% daily.
Get your tankwater tested for ammonia, nitrite and nitrate levels. If you don't have water test kits take a sample to the petshop and get them to test and give you the results. (only takes 5 minutes)