Niger's typically will need hiding spaces and need to put on a lot of size before they will come out. I've seen people suggest 10 inches or more. One method is feed the rest of the tank and then after lights out, drop some sinking food into the tank. Under normal circumstances, if it's gone in the morning, the Niger probably got it. Cut up earthworms, sinking pellets, small snails, slices of shrimp or fish, if correctly sized should get it's attention. Not sure if that will work with a much larger TSN in the tank.
With eels, small jars with narrow openings can be filled and left near the eel's pvc pipe. Not sure if Niger's will go for that.
You might want to grow him out some. I've seen people get them to more than 2 feet in between 18-24 months, so you should be able to get him from 3.5" to 9-10" in just a few months. Less competition, faster growth, plus you'll be sure it's eating, etc.
Anyway, I haven't raised one, so I hope you get some posts.