I got a about 5-6" ntt tiger almost a month. IT is still shy and not eating whatever I gave. I finally kick my 2 Peacock bass out in my tank in the weekend. How long he can survive without food?
My NTT that I got from the same LFS as you is now eating well. Here's what I did: I put it together with my baby motoro. Fed them everything except live feeders and the NTT didn't eat for two weeks, (but it kept watching the ray eat) only to hit frozen brine shrimp then spit it out. That was a good sign. I kept feeding that and eventually the NTT ate brine shrimp. The stingray was perfect because it gave the NTT an opportunity to eat before it hit the bottom, which was then cleaned up by the ray.
I used this same process for frozen shrimp and smelts.
This is my second NTT that I attribute the eating to my ray's "training".
What foods have you tried, and has it put anything in its mouth or spit out anything?
If he's too skinny, you'd have to force feed it (give pellets) or you would lose it.