{I will not} (ou thel(935c)). So many old manuscripts, though
the Vatican manuscript (B) has the order of the two sons
reversed. Logically the "I, sir" (eg(932c) kurie) suits better for
the second son (verse  30 ) with a reference to the blunt refusal
of the first. So also the manuscripts differ in verse  31 
between the first (ho pr(9374)os) and the last (ho husteros or
eschatos). But the one who actually did the will of the father
is the one who {repented and went} (metamel(8874)heis ap(886c)then).
This word really means "repent," to be sorry afterwards, and must
be sharply distinguished from the word metanoe(935c) used 34 times
in the N.T. as in  Mt 3:2  and metanoia used 24 times as in  Mt
3:8 . The verb metamelomai occurs in the N.T. only five times
( Mt 21:29,32  27:3  2Co 7:8  Heb 7:21  from  Ps 109:4 ). Paul
distinguishes sharply between mere sorrow and the act
"repentance" which he calls metanoian ( 2Co 7:9 ). In the case
of Judas ( Mt 27:3 ) it was mere remorse. Here the boy got sorry
for his stubborn refusal to obey his father and went and obeyed.
Godly sorrow leads to repentance (metanoian), but mere sorrow
is not repentance.