He is a human.. It's only the coating that making his p*nis not appearing...
It was Galactus who gives that coating to Silver Surfer... That silver like coating is a protection against coldness in the space...Silver Surfer is Galactus Herald who gives him(Galactus) food or power by absorbing the planet's energy force...
See this links:
www dot marveldirectory dot com/individuals/g/galactus.htm
www dot marveldirectory dot com/individuals/s/silversurfer.htm
Sorry for the dot words coz I can't allowed to post URL here...