Thank you very much for your insight! Do you have any advice to try to catch him? I just ordered Masstick!A mature 55G with a single mandarin will likely be able to sustain it pretty well on the natural population, so you do have time, especially since it is small.
That said, you should be able to get it eating prepared foods, and there are a few good directions to start. I seems like your choice of food is reasonable, but make sure you are feeding with at least the powerheads off (I feed with all pumps off), it drifting around doesn't really entice them at all, and they are methodical in their hunt to the point where they can't really chase anything fast or for much distance, so for it to settle and be recognizable it helps not to have flow going.
Next, I would make sure you are spot feeding it directly. It will probably shy away from you and your baster or whatnot initially, but gently send some its direction may help get the required attention. My mandarins are trained to eat frozen, but they sometimes have trouble recognizing foods they like around the tank after they've been sitting there for a bit, so presenting the fish with some food directly while it's watching is probably helpful.
Finally, if it's just not working, buy yourself a breeder box that holds onto the side (and I like using an algae magnet to further secure it), put a little cover in it (PVC fitting or the like), put the mandarin in the box, and feed it the foods it should like to eat there. It's a good way to make sure it eats, get it used to being fed rather than just picking around, and if it's forgotten or ignores foods it used to eat, the random picking around the box will definitely have it accidentally eating some of that food again. Having a much more confined space dramatically increases the chance that random picking eats some of the prepared foods. Feed the box preferably twice a day, until you can verify that shortly after feeding it eats the prepared stuff, and use a baster to clean out old food before the next feeding each time. I started from scratch with wild caught mandarins and each took about two weeks to train with this method.