If it is eating well that is a good sign and you have time to treat. Velvet is most commonly treated with a copper medication. I recommend Fritz Coppersafe, It is what ive always treated my fish with, including several species of trigger fish. First you need to do a freshwater bath to buy you some more time, Take some fresh water, must be dechlorinated, and heat it to the temp of the tank. Place the trigger into the fresh water for at least 3 minutes but no more than 5. I also recommend using an air stone in the fresh water to ensure proper oxygenation.
Get a qt tank of some sort up and running, a simple hob filter and a heater is all you need. You will need to get the copper up to 2.5ppm slowly but not too slowly. A reliable tester is necessary for this. I recommend the hanna copper checker. I would get the copper up to full strength over 24 hours. Triggers are tough and resilient and usually handle this well.