Sometimes when the tips of his fins are clear it might mean his fins are healing. Are you positive they're getting shorter?
But salt baths usually help, and a clean water EVERYDAY. Also, does he have white stuff growing on his body? If there is, it might be fungus fin rot and you can probably find fungus cure at the pet store. If not, it might mean it's bacterial fin rot. To tell the difference, fungus fin rot will be eat your fish's fins at a consistent area while bacterial will effect random areas. You'll also have to get different medicine for bacterial fin rot (maracyn-two worked well for me, but follow the instructions carefully!).
But if you've caught it in the early stages though, few days of salt baths a day (with a few hours between each bath) should be good enough.
To give a salt bath, you mix one teaspoon of aquarium salt in one gallon (4 liters) of treated water (or 1/2 a teaspoon in 1/2 gallon or 2 liters). Fill a small container (like a clean pickle jar) to the 1/3 mark with the salt water and just net your fish out and plop him in. Leave him in for 5 minutes than pour tank water to fill the remaining 2/3 of the container. Leave him in for another 10-15 minutes and then move him back to his tank. JUST MAKE SURE THE WATER IS THE SAME TEMPERATURE THROUGH OUT THE PROCEDURE. Changes in water temperature can make your fish go into shock and die. Also if your fish goes unconscious (starts floating on his side), just scoop him out and put him back into his tank.
This is advice given to me for when my betta had fin rot

and it worked well.