Seeing as in how Sid is a zero in almost anything he touches or does, the Prisoner may help him as a therapist with his problems, and move him up on the self esteem ladder.
Helping Sid with the mega-flamer, the sub-reactor, and whatever else he is working on, and actively teaching him along the way, will make Sid feel better about himself.
Character type completion breakdown
Combat Boy, Stealth Boy – Basically boils down to a Combat Boy being Sid's errand boy and listening to him bitch. If the Prisoner gives all the credit to Sid and is not condescending to him, Sid will feel like he accomplished a lot, and will feel better about himself.
Diplomacy Boy – The Prisoner has a chance to be Dr. Phil. A good Speech skill of any category can lead Sid into the realm of self-forgiveness and esteem.
Science Boy – If the Prisoner ends up doing most of the work but gives Sid all the credit, then Sid will earn some of Kyle's respect, which in turn makes Sid feel better.