Aim at a minimum of 100% water turnaround weekly. More w/c will only help. He does look a little skinny so feed a lot and clean the gravel a lot. Do not worry about the pH and temps more than keeping them consistent. Discus do not like change even current differences will aggravate my fish.
I sternly believe you can keep Discus alone or with other fish; they are kinda boring to watch alone. They will always do better in schools of 6+. You must be careful of disease. Yogias recommends deworming once a month. I do not do this unless my fish begin to look emaciated. All new Discus should be dewormed and watched for illness. Angels are ok with Discus but you must deworm those fish as they do not get affected as bad as Discus and you may not even know they have them.
Currently my show tank has 6 Discus (4 Leopard Snakeskins and 2 reds) 6 corys and 1 bulldog pleco.
OH YEAH! BUY YOUR FISH FROM A BREEDER!!!
What size is your show tank? I was aiming for a MINIMUM of 100% turnover weekly, prefferably 150%. I wish I knew where a discus breeder around here is... this is the only guy I have ever seen with discus, but his fish usually look quite nice. I'm not certain as to whether I'll go with 3 or 4... but I'm slightly inclined to lean towards 3. (hoping there will be no serious aggression... gonna keep cardinals and harlequins and hoping they will help dither)