I think the best way to introduce him would be in the after-credits of AoU, as a lead in to CA:CW.
There is a very quick, brief scene of a close up shot of Spidey swinging through the city, from the pov of Iron Man helmet cam, as the image pans from a close up shot, to a far away shot, revealing Iron Man standing on top of one, of the many, skyscrapers of Manhattan, watching Spidey swing by in the distance, then the face Iron Man, turns and looks at the camera, jets fire up, and he takes off, after Spidey. End scene.