Aquarium salt can cause constipation, bloating, and buoyancy issues. So it's possible that this is the cause of his buoyancy issues.
How much Aquarium salt are you adding? Is it possible for you to stop using it? (IMO, it's not needed as a preventative. So if there's no infection, you should be ok phasing it out. Just do frequent water changes to keep toxin and bacterial levels low.)
Yes, you can add Epsom salt directly to the tank. I would start with a dosage of 0.5 teaspoons per gallon. As Aquatail suggested, an easy way to do this is to put some of his tank water in a cup. Stir in 1 teaspoon Epsom salt, stirring until the salt is completely dissolved. Then, over the course of about 30 minutes, slowly pour the solution back into the tank. (This will provide a dosage of 0.5 tsp/gal.)
Then, when you're ready to do a water change, premix the solution, using 0.5 teaspoons Epsom salt per gallon of water. Add conditioner, and do the water change like you usually do.