Hi amanda. Sorry to hear about your guy being sick. The salts that you use when you do water changes, are they AQ salts or are they water conditioning salts that often available in the UK or Australia? If they are the water conditioning salts then that's a different thing from epsom salts.
You can get epsom salts at any drugstore or pharmacy. Just be sure they don't have any dyes or scents. The dosage you want is 3 tsps per gallon and if you can maintain the heat at 78-80 F that would be helpful too. While he is sick, increase the water changes to at least 75% once a week and redose the salt. The cleaner the water, the less likely he will contract any other diseases.
Because your betta is two years old, I'm guessing a lot of his condition has to do with his age. The average life span of a betta is 1-3 years and assuming your betta came from a pet store, he was most likely already at least 6 months old before you got him. Many of his symptoms sound like the types of conditions that occur with geriatric bettas, including the increased lethargy and refusal to eat. :(
Unfortunately, it wouldn't be a good idea to treat him with any medications right now because he isn't exhibiting any symptoms that would indicate which types of medications to try so using epsom salt is the safest course of action.
If you have any frozen foods like frozen bloodworms or brine shrimp, you can try those to see if he'll get his appetite back.
If he has trouble reaching the surface to breathe, lower the water level several inches. If necessary, you can lower it down to 2 inches. Just be sure to keep him as warm as possible.