Might be over reacting...

Housing
What size is your tank? 10 gallons
What temperature is your tank? 80 degrees steadily
Does your tank have a filter? Yes
Does your tank have an air stone or other type of aeration? No
Is your tank heated? Yes
What tank mates does your betta fish live with? One other tetra

Food
What type of food do you feed your betta fish?
How often do you feed your betta fish? Tetramin Betta medley

Maintenance
How often do you perform a water change? Once a week
What percentage of the water do you change when you perform a water change? 50%
What type of additives do you add to the water when you perform a water change? Conditioner and Ph stabilizer. I have well water.

Water Parameters:
Have you tested your water? If so, what are the following parameters? No, need to get a masters kit soon.

Ammonia:
Nitrite:
Nitrate:
pH:
Hardness:
Alkalinity:

Symptoms and Treatment
How has your betta fish's appearance changed? He as a spot on his top fin.
How has your betta fish's behavior changed? He is the same temperament as always.
When did you start noticing the symptoms? This afternoon.
Have you started treating your fish? If so, how? No.
Does your fish have any history of being ill? Not at all.
How old is your fish (approximately)? Close to a year.
It think it might possibly be lymphocystis but I'm not quite sure... I would like some more advice before I would start treatment.