Maybe these links could convince your dad
http://answers.yahoo.com/question/index?qid=20110507140258AA6Ym6Y
http://answers.yahoo.com/question/index?qid=20120509204755AAVUlhf
http://www.minecraftforum.net/topic/438951-is-minecraft-safe-to-buy/
http://www.debtordebt.com/forum/proof-that-minecraft-is-safe-t120740.html
And just search for sth like "Is buying minecraft safe" and you'll get many results: "So far 30,381,338 people have registered and 6,060,432 people bought the game." 6 million people bought it. Nobody claimed they have stolen their money.