That's a bigger hole than I realized. You may want to cover it, but you still want air to get in. Plastic mesh cut to size works well: http://www.shamrockcraft.com.au/MES3...-PC-%23/pd.php
You can get it a lot of places and it should be very cheap.
To treat this lip inflammation I would use Epsom salts - pure magnesium sulfate per the ingredients label with no other additives, perfumes or dyes of any kind. I know CVS Pharmacy has some: http://www.cvs.com/shop/product-deta...t?skuId=215202
As would most grocery stores. To treat you will predissolve 1 tsp per gallon (so 1 and 1/2 tsp for your tank size) - put a little tank water in a plastic cup and mix up the salts really well until they dissolve. Pour this dissolved mixture very slowly into the tank over the course of an hour. You must acclimate slowly because epsom salts can lower ph and you don't want to cause shock.
I would leave him in the salts for 2 weeks. I don't think he will need anything else, but just watch him carefully. They will not leave the water, so you only need to redose along with a water change. You should change his water more frequently during this time - normally 2-3 water changes a week are needed - 2 50% and one 100% weekly. During this treatment time I would up these water changes to 100% every other day. Use the guide back on the first page to do the changes without shocking.
ETA: Didn't realize you were using spring water.. that kind of water is usually RO and you actually can't use it pure. It lacks necessary electrolytes for fish health and the ph is usually far too low to keep fish in. It's really important whatever you use however to keep it extremely consistent.. no "about" mixtures as these swings can be really hard on the fish. You can cut tap water with RO at various mixture amounts - usually 50/50 works fine or is a good starting place, but you have to test your ph and gh to be sure.. You have to measure it exactly
the same every time, once you find a good mix. Usually using plain tap water is a lot easier, but there are rare cases when tap water isn't good. Can I ask why you don't want to use your tap water? Do you know the ph and gh of the tap water?