Shinji Ikari is a fourteen-year-old boy with an incredibly wimpy voice and also a very wimpy demeanor. This is amplified in this particular skit by the fact that he is portrayed as ill and has a very bad cough. I'd advise that you check out Spike Spencer's portrayal of Shinji Ikari if you want a voice to reference for your own portrayal, but honestly, if you can sound like a generic wimpy teenager, that's probably close enough.