POA "actor"

[clcb58]

I'm watching Prisoner of Azkaban (for the umpteenth time) and I just
have to ask, who is that kid that can't act that got all the key
lines? (Explains the Grim in Trelawny's class, comments on Sirius
Black's ability to escape the Dementors, etc.) I've checked the
credits and can't figure out who he must be kin to. He certainly
didn't get the part because of his acting skills. Anybody got any ideas?