I have a mandarin in a 57 gallon tank with very little live rock or pods. The mandarin was the only fish in the tank. I trained it to eat frozen brine shrimp with spirulina. It took about a week of feeding small amounts several times a day before he caught on. He is still a little skinny, but eating frozen food aggressively now. I think it helped that he did not have any competition for the food. Ideally, you would train the fish to eat prepared foods in a quarantine tank.
Not sure how long you have had him but,if he is skinny, then he is not getting the nutrition that he needs. You may be able to keep a mandarin in a 57 with plenty of LR and adding pods on a regular basis. But with very little lR in a 57 gal, his survival is and will be short term. I am sure many will disagree but ask how many have had them for 5 years or more. Also frozen brine shrimp is not a nutritious feeding. Just IMHO