We have a standard bike carrier attached to the back of our SUV and it sits on the carrier like a regular bike. We also have my husband's bike on the same carrier. No issues at all.

Yes, you will have total control of the bike, Steering, Brakes. The only thing you will feel is if your son leans quickly to the left or right, but normally you will be able to maintain the balance of the bike. My wife and I ride tandem’s, I know how to ride and my wife does not. You will be able to balance the bike without much notice that your son is on the bike. You will need to talk to your son about the team work needed to operate the bike but that will be easy with coast / pedal / shift commands. I think if you are riding with two it is more fun with a tandem bike, easy to talk and stay together.

Yes. It will hold your weight just fine. The frame is a high tensile steel, so there might be some flexing but probably not enough to really notice. Steel also means it can rust which will dramatically reduce the frame's strength. Ride on smooth surfaces, keep out of the rain, keep it clean, and you two will be fine.