First I used the eyedropper tool to find the exact colour of the t-shirt, then coloured over it.
step 2 of 3
I copied the cookie monsters face from his shirt, pasted it on top of the guys face, and re-sized it a bit. I then copied the left eye and pasted it on the right eye so they'd be the same. I also played with the exposure a bit.
step 3 of 3
Finally I pasted the guy's head where his shirt was, re-sized it, and changed the blend mode to 'vivid light' and voila!