If a Scout is not performing his job, it is the responsibility of the Scoutmaster to have the SPL remove him from that position. However, if a Scout is in a position for 6 months, and the Scoutmaster does nothing, the Scout should get credit for the position. That is in the R & R
I don't see anything wrong with telling a Scout up front he has to attend a % of meetings/activities to be considered as part of his job if it truely relates to his job (e.g., PL, or SPL). If he doesn't comply, remove him from the position.