The bed leaving sensor has been designed to detect frail, older people getting out of bed. It is quite sensitive, so if your son was very active in bed there could be some false alarms - for example, if he lifts his weight off the bed using his arms. This could act as an early warning system that he is active in bed (i.e. has woken up in the night).
The second option could be a floor pressure mat positioned next to the bed, which would only trigger when he actually stepped out of bed.
The third option could be a motion sensor positioned out of reach on the wall. The direction of the beam can be adjusted so that it only triggers when he leaves his bed.