They can be trained to take prepared food, which he will absolutely need to do in order to survive in a tank that small. As Monica says, he doesn't eat agae -- none of the dragonettes do.
Your best bet is hatch live baby brine shrimp for him as a starter food. Drain using a BBS net and rinse. Then mix with a little tank water and use a small tool to slowly and very gently squirt the bbs in front of him. Do this at least once daily at the same time(s) each day. He should eat the live BBS -- adult brine is really too big to get them interested although a good size specimen CAN eat them.
Once he is eating the BBS, start mixing in some dead/frozen BBS (You can freeze your own extras), and then start adding frozen foods like cyclopeeze and later frozen Formula One (a fine food for mandarins). Eventually, he should begin to recognize the feeding tool as FOOD and will come to it an start to eat whatever you squirt out of it. You can then start weaning him to a feeding station. (Google "Mandarin Diner" for ideas.)
The critical component here is consistency in timing and presentation of food with SLOW changes to the frozen.
If he is quite thin, Formula VHP is a good catch-up food, and it's similar to Formula One in ingredients and texture, so it should be easy to switch him off later.
If there are other fish that are aggressive eaters around, like clowns, you may not be able to train him, he will simply avoid the food because of the other fish. It is best to get fish on prepared foods in a QT situation. I would not move him at this point if you can avoid it. If your have something like clowns around that are eating well and preventing him to pursuing the live food, move THEM to QT for his training period.
