If he is not tightly attached to the rock, you can use a razor blade to cut the bysal threads connecting it to the rock. You obviously need to be very careful not to cut the foot itself. If when you touch it, if it is really sucked down, don't even try. If it is connected but you can tip it over cut just the threads where they connect to the rock.
Also, clams will often detach if they are shaded out and in a spot they don't like, so it might happen naturally.