It's possible to bring him back to decent health but, he'll likely not reach full size. The biggest challenge is in reversing the development of methemoglobin (nitrite poisoning) and restoring proper healthy hemoglobin cells. Fish do not have marrow to produce new blood cells. Their blood cells are nucleated and self-replicating. Clean well-aerated water will provide the environment for healthy blood cells to develop and overtake the unhealthy cells.
Next challenge is dealing with a lowered immune system. Again clean water and healthy diet will go a long way towards his recovery of a healthy immune system. Avoid supplements and vitamins for, at least, the oncoming year since his organs have been overtaxed and shouldn't be over-stressed by too many new items to regulate and metabolize.
For the next several months, it would be better to offer him multiple small portions of food instead of one or two large portions. This would allow the O the opportunity of regenerating a probably poor-condition and/or atrophied stomach and intestinal system.
Good save. I'm interested in seeing recovery details and pics.
what filter are you cycling?