Freak's ornithopter seems to be there in the most important moments. After all, it's the object that brought Freak and Max together, and it's what Max chooses to bring with him to the hospital to comfort Freak.
You're probably not surprised to hear that birds tend to represent freedom. But what might Freak want freedom from? Maybe his body. When he's first reaching for the bird, he can't get there because he's too small. He's also on crutches, which prevents him from doing some of the stuff kids like Max can do. But the second he gets on Max's shoulders, he's free. Together, these two boys can overcome anything.
And like Freak, the plastic bird has physical limitations. It snaps while Max and Freak are playing with it: "We keep doing that, it must be for almost an hour, until finally the elastic breaks" (3.17). This ominous bit of foreshadowing hints at Freak's own death to come at the end of the novel.
While Max is disturbed, thinking the toy is broken, Freak calmly says, "All mechanical objects require periodic maintenance" (3.17). There's that same calm and collected demeanor he has up until the end, when he tells Max, "I'll be busy getting used to my new bionic body. It'll probably take me weeks just learning how to walk with long legs" (23.38).