Oh those polypterus.
I used to have 3 of them, and getting them to eat when you first bring them home can be challenging.
I fed mine live fish, like rosy minnows. Those might be too big for your guy, so maybe guppy fry would work...or really small minnows.
After a while mine started to eat pellets, but I started out with live fish.
The only problem with live fish from stores (especially petco) is that they live in cramped conditions and if one gets a disease, then they all get it, thus leading to your fish getting that disease if you feed him those fish.
I didn't have any problem like that, but just be careful.