We dont have alot of information to work with, but there are two reasons why the aro isnt eating.
1. He isnt eating because he is stressed (tank conditions are not appropriate ie. he has outgrown his current tank, you dont have proper filter and water quality is bad, he is getting sick from this)
2. IF your tank conditions are fine, and he has enough space, quality water etc. then it might be simply that he isnt hungry. Arowanas are known sometimes to do this. The longest my Aro didnt want to eat for was almost 3 weeks at one point, and he was perfectly fine. Then he gets hungry and eats normally for a while until he goes on a little fast again. From my limited experience with Arowana, it seems to me they go through phases of eating and then fasting. for varying lengths of time. In the wild they dont have set meal times, I feed only when he is hungry and asks for food.
Please note I am talking about my experience with Asian and Jardini Arowana. I dont know if Silvers do the same (we dont even know what type of arowana you have yet).
Could you please give us some more details so we can help you answer your question better?
What kind of filtration do you have?
What size is your tank?