Accusative stays the same for masculine inanimate. For masculine (and plural) animate nouns, accusative matches the genitive. Мальчик is animate, therefore accusative matches the genitive мальчика. Incidentally, this is why "me" translates as "меня" rather than "я". I am (hopefully) animate, therefore accusative is the same as the genitive меня rather than nominative я.
P.S. Useful site: Enter a word here to see all its forms. http://starling.rinet.ru/cgi-bin/morphque.cgi?flags=endnnnnp