Okay I think the most important question is what kind of substrate (sand, gravel....) does he have in the tank? It is very common for arowana to develop this "droopy-eye syndrome", I have had some that came across it as well. The leading reason for this is actually the reflection of the fish in the glass (on the bottom of the tank). If there is nothing on the bottom of your tank, the arowana will look at themselves (quite vain, no?) and this will cause their eyes to point downwards. It is not necessarily BAD for this to occur, but it is no doubt unnatural and you should try to get something on the bottom. I have not kept many black arowana, but if you dont have anything on the base now I would suggest using very fine sand. No gravel or rocks, this can really injure them. As far as going back to normal is concerned, I have heard of the syndrome being remedied with a change in the substrate but it is sometimes permanent. Please update me on this, and if there is anything else I am pretty experienced with these fish so please ask...
dan