Ahhhh. That's quite old for a betta. In fact, it's the upper end of their average life span of 1-3 years. That makes things a little clearer. The little guy's age may be catching up to him. :( As with humans and mammals, as a betta ages, things start to slow down and things stop working like they used to. While in this state of advanced age, if he should get sick then he'll have a much harder time fighting off his illness. Just like how old folks often can't fight off common colds. :(
If you have any kind of salt that is non-iodized, you can use that as well but in general AQ salt is better because it doesn't contain anti-caking agents that could potentially harm him.
Aside from AQ salt? You might try API General Cure; if he has any kind of parasites - internal or external - General Cure will take care of them but also if he has a bacterial infection, it will provide some limited relief for that as well. And because it's not an antibiotic, we don't run as big a risk of developing resistances to the medication.
Come on, little guy! You've got to pull through so you can see your birthday!