Do not put salt in the tank until you think he is sick . And never add just a little bit though.
You don't need to use salt on regular basis until you need to treat him. Aquarium salt long term its doesn't prevent disease or parasites and long term use can cause resistant pathogens/parasites issues and kidney damage....and long term NON-therapeutic dosage can limit it ability to work for treatment when needed. So no salt yet. If he will show any symptoms of the sickness which he is not for now. And sometimes it aq salt or Epsom depending on the symptoms.
Aquarium salt for injuries,wounds,ext parasites,fin damage/rot (not plant tolerant)
Epsom salt for buoyancy issues,swim bladder problems,constipation, helps but not treat with internal parasites,pop eye,dropsy (plant tolerant)
Both have a antibacterial/fungal effect
Both can be use in doses of 1tsp/gall up to 3tsp/gall depending on what/why you treating . Always pre mix it and make sure it dissolved before you adding it . Do daily water changes if he is in the hospital tank (100%)
And if you have him with other fish i don't think other fish is tolerable to the recommended dosage you need to treat bettas. The therapeutic dosage differ from the the recommended dosage on the box instructions.
I am going to post a link about water changes. I am not recommending to do 25% water changes. Minimum of 50% water changes a week and i would do additional water changes since you have more than one fish. Also when you do water change try to make new water approximately the same temperature as in the tank.
I am posting the link for you about water changes, and it will be for one fish in the tank though.
I don't have filters in my tanks becaue it not cycled and i do 2-50% and 1-100% water changes for 2.5 gall tanks that i have. If you acclimate your betta properly it not stressful for them. All my bettas older than 2 years , and one of them 5 so ....So i think if you want your betta be happy more water changes is always good idea. And also bettas really more susceptible to the disease than other fish and from the reading many posts a lot of the time bettas get sick in the tank but all other fish doing good. So i am really recommending do not afraid to do more water changes. And make sure new water temp approx match the water in the tank