Title: The Orphan
Description: From the back cover of Timescape Books paperback March 1980:

THE BEAST PROWLED IN THE NIGHT, a wild-eyed, five clawed creature with a taste for blood.  But the strange ways of destiny changed his form.  When he found he could be human, he became little Robert, hid in a barn and was adopted by a farm family.

Robert had his own will and wanted to stay human.  Still the beast arose unbidden, shifting form without warning.  Neither beast nor child could conceal the danger, as their dual nature threatened to reveal itself, to destroy their cover -- and to leave them at the mercy of forces unknown!