He could take up the Dragon Disciple PrC.
Or he could find someone who knows the Ritual of Change from Savage Species.
I'd suggest either.
A magic item that turned you into a half-dragon, though? Cost? Uh.. lots. Like, more-money-than-you'll-ever-have kind of lots. I mean let's see here... stat bonuses across several stats, natural weapons, natural armor, hit die increase, breath weapon, energy immunity, resistance vs sleep and paralysis, darkvision, low-light vision. Think that's it. Ballpark figure would weigh in at several million gp, easily.