You should get him use to eating in front of you. Whatever you feed it, don't let it eat off the bottom. I had a 14" black arowana too. I did the same exact thing you did, let the food sit and walk away come back and the food is gone. I thought that black arowanas couldn't develop DE so I didn't care. Well, a few months later it developed DE in the left eye. I am 100% sure it was from bottom feeding because there was no other factors: no injuries to the head/eye from jumping.
