Feed it small feces! The dung beetle will produce fertilizer no matter the size of the feces you put inside his inventory! Using small feces weighs about .3LBs meaning you can stock his inventory extremely well and get tons of fert back.
Get yourselves a Phiomia as well! Feed it stimberries and you'll have plenty of medium feces to throw in the dung beetle. Fill it up, and come back and watch more fertilizer than you can imagine. I'll never use compost bins again!