You can try putting him in his cup or a tupperware container (don't fill it high enough that he can jump, or cover it with plastic wrap with a few holes poked in it) in 1 tsp/gallon salt. Float the container in his tank with the temperature up, so the temperature in the container will be the same. When I do this, I duct tape the container to the side of the tank to keep it floating and still (make sure to keep the tape out of the water). Pre-mix aquarium salt in a gallon jug. As long as it's the same water source and at the same temperature he's used to, it's okay to just net/cup him and put him in the salt water container.
So you can take him out of his tank and up the temperature, then acclimate him in his tank, then put him in the salt water. You can also just gradually up the temperature and leave him in the tank (about one degree an hour) then put him in the salt water.
If 1 tsp/gallon doesn't seem to be working, you can up it to 2, then 3, to a max of 5 (shouldn't need to do that, though). Make sure to change the water everyday in the container (to make this easier, I float two containers in the tank and switch the fish to the fresh one, then clean out the other and put water in it, and stick it back in). And remember, salt treatments for a maximum of 10 days only.