Usually cloudy eyes are a result of flukes that are attached either the outer lense or pupil area. If so I would treat with prazipro. Usually a 5-10 minute dip in FW can help confirm flukes although it will usually not effect flukes attached to the inner part of the eye. Take a small bucket of RO/DI water, match the temp and PH, dip him and monitor him for the course of the dip. Anything out of the normal behavior and I would remove him. Normal behavior for tangs and most fish will be very relaxed and swim a little then play dead and they will repeat it the entire dip.
FWIW, I monitor for 2 weeks then I FW dip for 5 min, then 5 days of prazi, monitor for 2-3 days then run 24 hours of prazi and add the fish. If anything happens that would suggest another treatment I then go ahead and proceed with that. I really overfeed during QT and fatten the fish up as much as possible. I actually add Methylene Blue to my FW dips just to help cure any fungus infections or aid in healing any open sores.
Good thing he is eating! Keep us posted!