Get the smallest frozen Brussel sprouts you can find,they may be called baby Brussels or something like that. Nice thing is they sink and the smaller ones are tender so he/she should be able to eat the whole thing. No leftovers to clean.
depends on which sp of plec it actually is, there are a few that get labelled "snowball", mainly LDA33 or L102/L201, LDA33 is a baryancistrus so should have a mainly veg diet, the others are hypancistrus and would fare better on a meaty diet