White spot on Betta's left side above fin. What is it?

It could be a fungal infection certainly, or it also could be simply a missing scale. It's hard to diagnose without seeing an image but are you 100% sure it's white spot. If it's either white spot or fungal growth it will be very hardy fix without being able to add medication. I recommend takign the Betta to an aquarium store or reputable pet shop and ask them to take a look at him. They can then give you exactly what he needs as they will be able to see the problem. If he's acting normal it could be simply a missing scale or the fact that he's having a latent reaction to a fungal growth or white spot disease.