Running off at swimming pool

My Son is nine and has ASC.  We go swimming as a family.every Saturday and he seems to really enjoy it.  Last week he suddenly got out and ran into the ladies changing room where two attendants kept him.until I came and got him/ he attempted to get out and run off again today.  He is a flight risk but the swimming pool.was always the place we felt he wouldn't try to run off.  It s one of his favourite activities at the moment, he asked to go swimming today Any ideas or adbuce?