Pre mix 2tsp/gall aquarium salt in one gall jug or in the container. I read very briefly all pages so not sure if you have him in the hospital tank? You will need to do 100% daily water changes. So it easier to do in smaller container or just lower the water in his tank.
Temperature over 85-86* will speed up parasites life cycle and help them to fall faster so you can remove them manually with water changes. So if you can raise the temp it will help.
I world take everything out of the tank and just wash it in the hot water and let it dry out for a few a week- it will kill parasites. No host they die.
Always rinse and dry out a net and changing container between uses it will kill parasites.
Aquarium salt you can use up to 14 days. Don't use salt less than 10 days.
I would buy medications in case you need to use it after salt. You always can return them if you don't have to use it.
Velvet will get worse with light. So if you have one for his tank don't use it.
When you do 100% water change make sure you introduce him slowly to the new water. Just add new water to his changing cup and let him get use to it for a few minutes and do it a few times.