My Fighting Fish is getting holes in his fins?

I have a Fighting fish in a 28cm x 10cm tank. It has pebbles down the bottom, a plant to the left and a aqua nova air pump attached to filter to the right. He was happy and healthy and we have had it for about 5 weeks in which we have changed the water once. He is feed blood worms every night. He first started getting dirty gray colour spots up on his head and just to before his fins. Then he started getting holes in his tail and now I have noticed that his fins are also missing half of the original size. His tails and fins are also looking frail and very thin. I would love to know what is wrong with him and if its fixable. Any questions please ask!