My first Idea was, to use a micro switch and I came back to this Idea.

I just changed the design, to make it simpler to use and my son will not detect that we there is something manipulated by us. I analysed, what happens, when he removes his pants, and I saw on the surveillance video, when he removes his clothes or when he goes to the toilet, that he has to pull sideways on the pants to remove them, so I decided to try something with a pull switch.

Seems to work.

I use a micro switch, and put a small protection over it, so that the cord will not be blocked in the foam and fabric. A small piece of PVC bend with heat will do the job.