patience i guess... I have a similar thing going with my msobo. I have a male who has the top and bottoms of his fins turning blue, but he has stayed this way for a while. He is between 2.5-3 inches. I am leaning to 3 more so. Boils down to patience, as I dont see anything i do changing it...
I must agree with everyone, your case is super strange. I have seen young johanni at the lfs already starting their change... I forget what u said is stock, but maybe take a 30 gallon tank and set it up for species only short term to encourage breeding and see if he will complete his change. If he does, u know it is due to the tankmates