You are sure the thermometer is correct-if so, unplug the heater and get a zip-lock bag and put a few ice cubes in it and place in the tank until the temp is equal to the water temp he is in and then plug the heater back in...or better yet...take it back and get a 25w heater
You have to be careful with sudden high temp with an already compromised fish or you can end up with an even sicker fish
Although, 85-86F is what you want to speed up the life cycle of the parasite..it may not be the best for the fish at this point since he is already weak to start
You can also do a two cup method in mild cases of ich-with or without aquarium salt 1-2tsp/gal-use two cup and change from cup to cup each day-dump the old cup and fill with fresh dechlorinated water before transfer each day-continue this for at least 3 days past the last day you could see the parasite on the fish.
About how many white dots are on the fish and what part of the body can you see them, how is he eating......