Shape Shifter — Chris Archer

In the fifth book of this middle grade sci-fi series, a teen abducted by aliens becomes a shape shifter, unrecognizable to everyone, even himself.
 
For nine months, Todd Aldridge was missing. Stories swirled about him throughout his hometown of Metier, Wisconsin. Was he kidnapped? Abducted by aliens? The kids at his junior high school were keen on the alien story. Metier is a UFO hotspot, after all. Until one day, Todd is found alive beside the town reservoir. Everyone wants to know where the thirteen-year-old has been. Only Todd doesn't remember anything, except for the light that filled the sky moments before he disappeared . . .
 
Now Todd is beginning to wonder what happened. Especially when some of his classmates claim that he is an alien now -- like them. Todd isn't ready to believe them. Until he feels the power surging in his body and his shape beginning to shift. He morphs into a creature that he can only call otherworldly, a creature who will have to fight to survive the danger surrounding him . . .