First, you will need to write your screen PBO module such that if any of the checkboxes is set to ‘X’, the related field is displayed and if the checkbox field is clear, you hide the related field. You do this inside the LOOP AT SCREEN..ENDLOOP block.
Then, when defining your checkbox in the screen editor, you will need to assign a dummy function code to that field. You do not need to handle that function code. All this function code does is trigger the PAI and PBO when a user checks or unchecks the checkbox.