My sincere apologies. I did not see that you'd filled out the template & quoted it. I probably sounded snippy & I apologize if I came across as rude. Not my intention but re-reading, I may have come across that way.
The advice remains the same tho, especially as you have live plants & maybe cory - both sensitive to salt & some meds. Looking at your tank descriptions in your profile, you had cory in this tank too. Do they still share the tank?
If the white spots are new, plus the new lethargy, I'd suspect either
ich or fungus (if they're fuzzy spots) as you do. From what I understand there's some mighty heat treatment resistant ich in Australia, so you'd be reduced to salt or proper meds if you're seeing the spots multiply.
I'd recommend
aquarium salt treatment for either issue & if it becomes severe,
malachite green. Because of the plants & cory (if they're in there), I'd set up a hospital tank or bucket with heater & filter for the betta. 1.5 tsp pre-dissolved aquarium salt per gallon, 50% water change after 2 days, then add 1 tsp aquarium salt per gallon to tank, wait 2 days, 50% water change & add .5 tsp aquarium salt per gallon, wait 2 days & do 50% water change & leave it. With hope, the spots will be disappearing or gone by then & he can be returned to his regular tank. The goal is to slow or stop the ich/fungal growth to give the animal's immune system a chance to kick in & do the rest.
If there is no change or more spots appear as you go along with salt treatment, I'd move the fish back into his tank (or he can remain in the hospital tank) & use malachite green. Not sure how it comes in Australia but I normally use it as a product from Kordan's marketed for ich & fin rot. There are good instructions on the bottle for 3 day treatment with water changes between. In my neck of the woods it can be purchased as straight malachite green too, with similar instructions. Warning that malachite green will stain, including
silicone seams in glass tanks.
Too, while I do not combine the two, in severe cases of fungal or ich infections, there are those who combine both salt & malachite green treatments at once. I prefer to start with the least intrusive med (salt), rather than combining treatments.
I hope he is showing improvement with the super clean water but if he is not getting better or getting worse, I recommend the aquarium salt &/or malachite green treatments above. If you have questions, please do come back for help & keep us updated. He's a beautiful animal & I hope he does well.
Again, my apologies for sounding impatient & being less than fully helpful, not seeing the completed template.
Hope it helps!