You'll want to get a good shot down his mouth. What has he been eating? At a guess, the salinity drop is making him unhappy--you may notice a darker tummy, and less 'bright' coloration as well. Puffers are tough to treat for skin ailments, but increased temp and salinity usually works. If you haven't, I'd bump the temp up to about 86-87, keep it there for a week, and start returning the salinity to normal right now. The temp will take care of the parasites as well, and the return to 'normal' salinity should get the puffer eating again.