Did you quarantine the female, if you didn't chances are your resident may have picked something up of her which is stressing it.
All new editions should always be quarantined for minimum of 30 days to ensure that they are healthy and won't pass anything on to current residents.
You should really remove her from the tank and quarantine her; but as he appears to be sick; it is probably a better idea to remove him from the tank completely and keep him in a temporary tank of some sort - bucket or plastic container in dechlorinated water, as long as the water temperature is 20C or below (lower than 20C is better as heat can add to stress).
What is your tank temperature - anything above 20C and you'll need to cool the tank down using some of the following methods
http://www.caudata.org/cc/articles/cooling.shtml
How big is the female - really two axies in a two foot tank can be rather cramped fouls the tank faster - the tank temperature in a smaller tank increases faster than in a larger tank - especially if you're using an internal filter.
You need to test the tankwater regularly (usually weekly or fortnightly before waterchanges, especially with additions of other companions - their waste increases the bioload). If you don't have freshwater test kits, test tube kind not the all in ones, then take a sample of tankwater to the petshop and ask them to test for ammonia, nitrite and nitrates. If ammonia or nitrite levels are above 0, then you need to do frequent partial waterchanges (no more than 20-30% - too large a waterchange can cause your tank to cycle again and may cause more harm than anything else).
As you do waterchanges twice a week anyway, I think your problem may be more related to addition of unquarantined animal and heat related stress.
Table salts can be used if you don't have aquarium/tonic salt or sea salt.
Salt bath method is:
Use a plastic container (with lid) - icecream or other that fits him lengthwise.
Fill the container with enough dechlorinated water (tapwater treated with water ager/conditioner/dechlorinater drops to remove chlorine and chloramine from the tapwater).
Use 2-3 teaspoons of salt per litre (or 2 pints) of dechlorinated water, stir the salt till it dissolves. If you have tonic/aquarium or some sea salts you need to crush them to dissolve them - they're quite chunky.
Immerse your axolotl in the saltbath, cover with lid and leave him in it for 10-15minutes maximum (no longer as salt will burn gills).
Remove him from saltbath and return him to tank/temporary container (wherever you decide to put him) and empty the saltbath.
Saltbaths need to be done twice a day (3 times a day if you're able to) and continued at least a week after the fungus has dropped off.
EDIT: One thing, do not add anything to the tank to fix it, especially if advised by petshop. Only thing that should be added if you use tapwater for waterchanges is the dechlorinater drops.