I am wondering why you think he has velvet? I would think if he would has velvet for that long he probably would not be alive.
google the velvet , fish will look like sprinkle with salt or
Velvet is hard to spot, but can be best spotted with a flashlight. Shine the light on the betta’s body: if it looks like it is covered with a fine gold or rust mist, then it has velvet. A betta with velvet will act sick, so look for clamped fins, scratching against rocks/gravel/tank, loss of appetite, loss of color etc...
If you really he has velvet you need aquarium salt and have medications on hands.
You are right aquarium salt can cause kidney damage and other problem but only if you use it long time and not therapeutic dosages. If you need to treat fish it one of the natural remedy method that you can use to help your betta. And you can use it only for certain period of time.
So check those symptoms and if he really has it then we will instruct you with directions.
If he has velvet i would recommend 2tso/gall of the aq salt. And you will need to do continue to do daily 100% water changes with salt. And it always better to pre mix salt for the right dosage. Of course with betta acclimation.
If you decide he has velvet (external parasites) i would also buy medications for external parasites.
With aq salt you can treat maximum for 13 days, and for external parasites you need to treat him pass 3 days since you stop seeing them ...
What do you have in his tank? If you have any decorations , gravel i would take it out.
Give us update i will instruct you on what to do with decorations if he has parasites.
Also i will give you instructions on the nets or anything you are using between changes....you need to dry out so parasites die.....