Diagnose My Betta?

[Tazziania]

Went away for a week and returned to Gilbert looking like this. He's in a 28 litre tank alone. Water temp in set to 25c. Tested the water and no nitrates.
For the past 5days I've been changing his water daily and adding aquarium salt because I first thought it was fin rot but now I'm not so sure as there has been no change to him at all. He's still eating and moving well. Isn't spending too much time on the bottom of the tank, nor at the top. I'm well and truly stumped. I've only had him for about 2 months so I don't think its age related... Help?!

 

[LyraGuppi]

25C is too cold for a betta, they prefer somewhere around 26 or, more preferably, 27C.
 
Do you have a picture to help us?

 

[Meeresstille]

Could you tell us if his tank is filtered and what your water changing routine is?
 
It is also possible he may have been bored during your week away and started to bite his fins. 
To help fin re-growth you can add stress coat to the water. 
Adding an Indian Almond Leaf can also help.
 
[SIZE=14.4444446563721px]You can increase the aquarium salt to 2 tsp per gallon, but don't go longer than 10 days with aquarium salt treatment. [/SIZE]If it is fin rot and the aquarium salt is not working you will need medication to treat him.

 

[Wildbetta]

A picture would definitely be needed to be able to tell what might be the issue with your betta.  I have found a lot of people think their bettas have sudden onset of fin rot only to find out the fish is biting its own fins. 
 
In the meantime, bump the heat up as your temperature is a bit on the chilly side for a betta. 

 

[skink_boy]

Increase the temperature sudden changes of temperature kills fungal and bacterial infections.