Someone else has posted about this book years ago and it went unsolved, but they left out some details that I remember and hope will jog someone's memory.
Here's their post: https://www.reddit.com/r/whatsthatbook/comments/zn8j7t/juvenile_chapter_book_fantasyscifi_with_metal/ "I only have vague memories of this book, to the point I've started doubting if it exists. I'm not sure if it's sci-fi or fantasy but I think it's one of the two. From what I remember it's about a boy and at the start of the book he follows a bird (maybe a hummingbird?) which is made of metal. Somehow this bird leads him to another world. I think the middle of the book is just his adventures in the world. I have vague memories of there being a female character who was his companion during his travel? At one point his eye is replaced my a more technologically advanced fake eye. At the end of the book, something happens and he returns to his normal world and originally thinks none of it ever happened, till he accidentally takes a picture with his fake eye which makes him realize it was all real. I think the first and last scene may both take place on a school bus? Any help would be appreciated!"
The details I also remember is that in the other world, the main character integrates into a society of people that are terrified of some kind of cyborg plague that could corrupt everyone if it is let in. They have some kind of testing device to figure out if the blood of a person has been infected with the robot virus. I think the boy pretty immediately is revealed to have the cyborg plague? That is how his eye ends up becoming higher tech, but he ends up being able to control his powers instead of turning evil. Before he returns to his original world, at the climax of the book he fights like the super terrifying cyborg villain who wanted to take over the other world, and the main character ends up using his own cyborg powers to take control of like the thrusters of some kind of defunct spaceship that was in the junkyard they were fighting in and immolates the big bad in the thruster fire, or at least I think that is the case. I know they fought in a scrapyard and were using their cyborg powers to control the scrapped machines there. Been trying to find this book off and on for years, hopefully yall are able to help!