For the past several weeks he has donned a cape and ran around the house chasing his younger brother and threatening him a monotone robotic voice.
Should I encourage it?
Not only should you encourage him/her, you should insist the school provide a restroom for transformer robots.
Of course you should encourage him, and not repress his self-chosen species identity. /s However, there’s a significant chance that if you encourage this, when he grows up he’ll end up in the Air Force, with an aeronautical, aerospace, or mechanical engineering degree, and he’ll still have a deep emotional attachment to his voice-changing Optimus Prime helmet.