pater: such a preëmiuent type of starvation is Aurelius that he might well pose as the parent, or presiding genius, among all similarly afflicted persons: cf. Mart. 12.53.10 “huic semper vitio [rapacitati] pater fuisti.”
esuritionum: the word apparently occurs first in Catullus (cf. also Catul. 23.14); it is also found in Petronius and Martial. With the use of abstract for concrete, cf. Catul. 47.2 “scabies famesque mundi” , and often.
 discet: Aurelius is pater esuritionum (Catul. 21.1), and the boy will of course be taught bad habits by him; i.e. if the affection of Juventius is won away from Catullus so that the boy will not return to him, but prefers to live as the protégé of Aurelius, he will perforce have to share the privations that exist in the house of Aurelius. It sounds as if the poem were meant to toll back Juventius as much as to score Aurelius.