If he is suffering, maybe it is time to humanely euthanise.
I am currently facing the same situation with 8 or so fry who didn't have proper nutrition, picked up internal parasites, and were exposed to persistent low levels of ammonia during the first few weeks of life.
They are still severely stunted at around 6 months, and it is obvious they will never thrive in spite of my now fastidious cleaning and feeding regime. One of my biggest has now come down with dropsy and I found two dead fry the other day.
I am thinking I might just save the only almost grown fry and cull the rest. It is obvious they have suffered the effects from their early bad start, and I think it's kinder to end it than to watch their numbers slowly dwindle.
It's a very hard decision to make, but if the quality of life isn't there (Spartan doesn't swim much, doesn't eat much and is much much smaller than he should be at this point in his development), it might be time to do the responsible thing and euthanise.