You may feed them shirmp, tilapia fillet, superworms, earthworms, frogs, crickets, nightcrawler, tubiflex(for juvenile) or dry fish foods which are made for arowana. It's all depend on the availabily on your area. Just don't stick on only one type of food.
Try bloodworms, brineshrimp, and if it is able too regular fish food. Not all arowanas will eat regular fish food but some do. Also small crickets, all of them should eat crickets. Then later maybe small goldfish or guppies. My silver aro would only eat bloodworms, goldfish, crickets, and other live worms as a baby. Now he is big and will eat almost anything good luck.
If you wanna feed him some insects, try lobster roaches my arow goes mental for them and hed be able to tackle medium sized ones at that size. My jardini loves his roaches.