Good technique to superimpose face into moving character in AfterEffects?
There is some substantial work that would go in an effect like that.

You'd have to first track the face of the slowed-down singer in 3D-space and apply a mesh to it, which sticks perfectly to his features.
Then, you'd want to rotoscope the normal-speed singer's face (which is going to take a lot of time alone) to then apply that pass with your tracked transform to the slowed-down background. You'd also have to match the change in shape of the head, such as when opening the jaw. This is not only applying to the face, but the whole head really.

(If I understand your vision correctly,) this is a high-end job that would take a few weeks for a professional compositor.

I would recommend you film the entire background in slow-motion, then shoot the singer in front of a green-screen so you don't have to rotoscope him out of the background. You can then simply key your singer in normal speed, track your slowed-down background (if there's movement in the camera) and place your singer on your background, applying the motion of the camera.
This method is not a walk in the park, but significantly easier than the approach above.

Hope I could help, have a great day :)