I think it'll turn out pretty much like this:
Chap thinks he's up and coming boxer, tells everyone he's going to win his next fight and it'll make him famous, including his love interest at bar/diner. Loses fight, all embarrassed, swears he'll quite boxing. Chap approaches him, offers him one last chance to make it big, our wanna be boxer humms and ahhs a bit and after talking it over with his diner lady and / or brother, (his brother trained him so he feels bad about leaving him behind) decides to give it a go.
Starts doing well, wins a load of fights etc when brother starts acting out, gets arrested, gets hand broken, falls out with brother, not sure if it will recover in time for championship fight.
After talking to diner girl forgives brother, hand mends just in time for championship fight. Starts off well in final fight, then starts taking a beating, sees g/f & brother in audience, gets second wind, wins fight.