DO NOT PUT DISCUS IN 3.5 pH!!!!!
Discus do not like change and can thrive in a variety of pH levels. Consistency is the key. My Discus are in about 7.7pH and they are doing great. pH from 6-8 will be ok for Discus as long as you keep it consistent. You can lower it if you isolate a breeding pair and wish to induce spawning. Peat moss is used for this.
Your fish is stunted man. Isolate him do lots of water changes and feed him as much as possible.