Even if the kid survived to the point of liftoff, he would have been dead upon entry into Earth Orbit. Apollo used a low pressure, pure oxygen atmosphere to save weight, and cabin pressure dropped from 16.7 psi to 5.8 psi in less than 2 minutes after launch. To avoid getting the bends, the astronauts had to breathe pure oxygen for about 3 hours prior to launch. This is why they walked to the spacecraft already sealed in their spacesuits. To reduce flammability, the cabin was pressurized with a 60/40 mixture of oxygen and nitrogen, which would not have adequately prepared the kid for the pressure drop.
There was no capability to measure vehicle weight before launch. Even if there were, it is hard to believe it would be accurate enough to detect the kid's 90 LB weight within the 6.4 Million LB weight of the vehicle. Furthermore, vehicle weight *would* fluctuate more than the kid's weight, for many technical reasons.
The kid would have died from suffocation before launch. The only means of carbon dioxide removal was with the Suit Circulation Loop, within which the three astronauts were fully sealed. With all suits sealed, and the "CABIN AIR RECIRC" Valve CLOSED (per checklist), the kid would have suffered the same fate as if trapped in an old refrigerator.
Once the kid's presence had been discovered, completing the mission would have been out of the question. There was always the possibility that the LM might fail to re-dock with the CSM - the alternate plan was an EVA (spacewalk) transfer. Since the kid had no spacesuit, this would have been fatal to him.
There would have been no advantage to operating the air scrubber (i.e. carbon dioxide removal) at a lower level. Elevated carbon dioxide levels actually cause a person to use more oxygen.