Thanks everyone. His fins have actually gone worse since the original picture. He's beginning to look more like a crown tail than a veil tail
I have attached a few more pics, sorry they aren't very clear as my little guy is very active its hard to get a good shot of him.
Ive been adding salt as suggested. I do partial water changes twice a week at the moment. I have removed some rocks to make sure nothing sharp could be damaging his fins. All that is in the tank now is live plants and a few rounded white stones. The filter is a sponge filter powered by an air pump, there is nothing on it that his fins would get caught in.
From what I have googled it doesn't look like fin rot, but I'm fairly new to this so I may be wrong. My fish is very active, he doesn't appear stressed, he eats fine. I feed him once a day, 6 times a week ( Atison's Betta Pro and New Life Spectrum Thera A pellets ). I have never witnessed him biting his own fins, my water parameters are fine, so what am I doing wrong? Shall I dose with fin rot medication?