NTT datnoid feeding problem..

First I'll say I've never had your exact situation as I've never had a fish I couldn't break off live; so I guess my advice should be taken with a grain of salt. I have however had fish in a very weak condition due to not eating who where in a condition similar if not worse then your Dat. In my case it was recently imported Flowerhorns and Red Mammons who had intestinal infestations. So I treated them with a epsom and pellet solution using a pipette. Which if you feel comfortable doing force feeding with the pipette is an option. This however is more then likely not a recourse you'll be comfortable with. So there are two other options.

One would be moving the Dat to a small hospital tank(10gal or so depending on the Dats Size) or using a breeder box that attaches to the inside of your tank. After acclimating to the new accommodations the Dat will have a much easier time feeding on live due to the confined space.

Another would be to hobble the fish to make it easier to catch but I wouldn't try and stun the fish. Stunning them without actually killing them or putting them in a paralyzed state will probably be very hard. Since your Dat only eats live more then likely he'll ignore a unmoving fish, which is the state most of the fish you try to stun will be in. Your best bet and again I'm not promoting such behavior in general but if you feel you have no other choice is to try clipping the feeders tail, the soft rays. This should hopefully slow them down and make them clumsy enough to be caught.