Depends on how he stretches, I guess?! Anything from the Puppet tool to other distortion effects to re-creating it in a 3D program. Without seeing any reference, this is slightly pointless, anyways. Too many ways too skin a cat. Provide some reference screenshot and we may be able to provide better guidance.
One of the important steps will be separating the subject (the person) from the background, so that you can distort just the person. This means that you'll need to shoot him against a green screen or do some rotoscoping---preferably the former. (See this page for basic compositing information.)
As Mylenium says, it's hard to know what to suggest beyond that without some more details about what you are going for. For example, the image will degreade quite badly of you're stretching a small number of pixels in the original arm across a very large distance. If you're stretching very much, a simple application of a Distort effect won't be enough.