Can you get a clear and closeup picture of the "sores?"
If they are clear and bubble-like, it is probably ick. You would treat this with 6 weeks hyposalinity in a QT. This is an easy treatment, but you must be meticulous. While mandarins are resistant to ick, they are not immune.
If the sores are pink or red and open, it is probably a bacterial infection of a wound. If mild, it may cure on it's own. Your changes here are greatly improved if he can be placed in a quiet, low lit QT with plenty of live food. Or you may treat in QT with an antibiotic which covers both gram-negative and gram-positive bacteria (since you won't be ID'ing the bacteria) like Furan-2. If in doubt, do a double-length treatment (8 days). Furan-2 is readily available, but there are other brands. The key ingredients are 60mg nitrofurazone and 25mg furazolidone, plus usually a blue or green tint for the water.
Other possibilities are a fistula (relatively rare and not always treatable - high doses of antibiotics and a quiet QT with lots and lots of enriched foods would be the prescribed protocol.) Or, it may be an exit would from an internal parasite, in which case you would treat with the appropriate anti-parasite med for the parasite type.
The clear, focused and close up picture is really needed to identify the illness.
Personally, while at the store I would stock your fish cabinet with a package of each type of med (about 4 main kinds.) They are cheap. Then, also get some BBS eggs and an airpump, airline, backwash valve and airstones for hatching them. An enrichment media like Selco/Selcon will help make the BBS more nutritious if you end up keeping a fish on them for a long time. For example, a mandarin in QT who is untrained on frozen foods yet.
