I think it would help to use aquarium salt, and keep him out of view of the new betta.
I hope he gets better soon!
Does he have a heater?
If it is one that can be adjusted, setting to the lower to mid 80's would really help, but it needs to be done slowly. Let him adjust to it.
Indian almond leaves are known to really help bettas, and provide them a more natural environment. I just got some and I put them in yesterday.
You can get a water conditioner with indian almond leaf extract, at petsmart. It's made for bettas, but It will leave white particles in the water.
You've had him for 2 years?