Is he stable enough to do a complete change? Otherwise . . . another option is to get a 1 gal jug and fill it with 5 cups dechlorinated water. When you get the meds, you'll add the first dose to that. I think it is 1 packet. Mix well. Next, get another 1 gal jug or something that holds 1 gallon of water. Fill that with dechlorinated water. Now pour 1/2 cup of the medicated water into this jug. Then fill up his cup with the now medicated water. I know, it's a lot of steps but it's the only safe way to medicate small tanks.
Now, if you ARE able to do a 100% change and remove all the salt and you are going to put him back into the big tank, then you can still use the 5 cup method for preparing the medicine. However, add 1/2 cup of the medicated water for every gallon of his tank. So he would get 1 cup and 1/4 of the medicated water.
I hope I'm not being too confusing. I kinda confused myself there.