With hormones it's quite possible that the color loss process could take a few months in total, especially if stronger doses of hormones are given. I recall back when Denis at Gold Aquariums first imported some RT's approx 6-7 years ago, color was amazing, blood red with white as snow pearls, until the juice wore off several weeks after importing them. His $300 fish went to $0 in a hurry. Ouch.
If it's not due to hormones, then I would stick to what Duane originally suggested, get him back into the same setting as he was when he was showing his best color. Black background with dark blue gravel, and cross your fingers. There's plenty of asaxanthin in his diet (which enhances the color red), so diet won't be the issue. Good luck.