Here are instructions for fin rot but if he lost his appitite I think there may be something else going on with him.
Do you have a clear picture? Many times tail rot and tail bitting get confused. If he is bitting his tail, he can get fin rot. However, I have never had a betta with fin rot stop eatting. Also fin rot can be a symptom of something else...but we need more info to tell. Look at him very closely, are there any fuzzy patches? does he look like he has grains of salt on him? If you have a flashlight, shine it on him. Does he look like he is sprinkled in gold dust?
How big is your tank? If you turned the filter off and then back on again - any of the good bacteria would of died in the 5 days it was off. When you turned it back on, the dead bacteria could of caused an ammonia spike. If its under 5 gallons, I have heard the filter can't actually cycle so I don't know how that would affect things.
Also do you have a heater? If they are cold (under 78 degrese) they can lose color and become lethargic.
Bloodworms should be given as a treat only - pellets are best. Flakes and freeze dried foods can lead to bloating/constipation. Frozen bloodworms are better but I think they can cause parasites?
Tail rot or fin rot
•Symptoms: Betta’s fins and/or tail seem to be getting shorter and shorter or they seem to be falling apart and dissolving, Black or red along the edge of the betta’s fins/tail, Bloody tips, Behavior may not change
Treatment: Conservative: Treat with Aq.Salt at 1 tsp/gal. Increase water changes to 100% daily. Replace accurate amount of salt following water changes. Add Stress Coat to help repair tissue. If there is little to no improvement within the first 5 days, you can increase the salt dosage gradually to 2tsp/gal but do not continue any salt treatments past 10 days. Medication: If Conservative treatment is ineffective use API Tetracycline, API Fungus Cure, API Triple Sulfa, OR API Erythromycin. Also add Stress Coat to help regrowth. Continue until fins/tail stop receding and start showing some new growth.
can you answer:
What size is your tank?
What temperature is your tank?
Does your tank have a filter?
Does your tank have an air stone or other type of aeration?
Is your tank heated?
What tank mates does your betta fish live with?
What type of food do you feed your betta fish?
How often do you feed your betta fish?
How often do you perform a water change?
What percentage of the water do you change when you perform a water change?
What type of additives do you add to the water when you perform a water change?
Have you tested your water? If so, what are the following parameters?
Symptoms and Treatment
How has your betta fish's appearance changed?
How has your betta fish's behavior changed?
When did you start noticing the symptoms?
Have you started treating your fish? If so, how?
Does your fish have any history of being ill?
How old is your fish (approximately)?