I had a mandarin dragonet (despite the popular name, it is not, in fact, a goby) who lived for three years in my tank. Copepods are the best food for Mandarins. Even with tons of live rock, 32 gallons is probably not big enough to sustain a breeding population of copepods, so they must be supplemented. I prefer the copepods from Algae Barn (and if you order the right pack on Amazon, it also includes a bottle of phytoplankton, which your mandarin will eat).
With a Mandarin in a 32 gallon tank, you probably will not be able to rely on the copepod breeding population in the tank, so you have to supplement copepods. As you are doing the copepods, start introducing other food they like (see recommendations above), and they will begin to eat this so that they are not entirely dependent on copepods and having to restock copepods in your tank regularly.
I would recommend start with copepods, then slowly introduce other foods they like, and eventually they may become less dependent on copepods, so his life won't depend on just one food source.