My best guess without actually seeing the situation would be that your nephew may be feeling anxious/fearful when he first greets a dog (after being bitten so many times). Perhaps Jake, already feeling stressed, sensed this anxiety and saw it as a threat.

It'd be really hard to know without knowing every bit of the circumstance, and it also does seem unusual for a child to have been bitten so many times. I wish I knew what to tell you.