Hi! There are more experience discus keepers here than me.
But I have raised small discus to decent sizes. I would advice you what worked for me. If any of my discus stopped eating, First thing I would do is to increase the temperature. You can increase it to 88 to 90 over time (not suddenly because it will shock them) and keep it for atleast 1 to 2 weeks. This should cause him to start eating.
Higher temperature increases metabolism and enables them to fight whatever is bothering them. It's great you have noticed it early. You can start with the temp. treatment. If it doesn't work we can move to more complicated treatments.