You can find him hiding behind a tree beside the cinema. Approach him to get his friend request and this sidequest. The locations where you need to place the sensors are marked in your map.
Sensor Placement Locations
Fire danger meter west of South Park Elementary
Wind Cockerel, rooftop of the barn
Electric transformer, in the roof of Kenny's garage. You can unlock during the main quest Hot Coffee.
Return to Al Gore to receive Al Gore's Storage Key. Go to U-Stor-It and open the storage shed to the left of Macky's storage. You'll receive the ManBearPig Defilibrator.
Next, head to the sewer and use the alien probe to beam yourself to the other side. Take the ladder down then shoot the valve to moderate the sewage flow. Go past it and operate the yellow valve to stop it completely. Next, place the Defilibrator on the generator nearby. You now have to return to Al Gore to report the readings and complete the quest. However, you can explore the sewers further if you wish. Refer to the sidequest “Dropping the Kids Off” for the complete walkthrough of the sewers.