How to show a simple Message Box in APEX

I'm trying to show a confirmation Message Dialog box in my screen after an IF validation results as true.

PL/SQL Condition.

        IF LV_SOURCE = 'SoS' THEN
:P3600_SHOW_GRIDS := 0;
ELSIF LV_SOURCE ='Bin' THEN
    :P3600_SHOW_GRIDS := -1;
ELSE
    clearInfo;
    :P3600_MESSAGE := LV_ERROR;
END IF;
EXCEPTION
    WHEN OTHERS THEN
        clearInfo;
        :P3600_MESSAGE := REPLACE(SQLERRM, '''', '');
END;

JavaScript condition to show or hide a grid

$(document).ready(function(){
$("#P3600_LOTID").keypress(function(event){
    var k = event.keyCode || event.which;
    if( k == 13 && $(this).val().replace(" ", "") != "" ){
        apex.submit({request:'P3600_LOTID', showWait: true});
        return false;
    }
});

if( $("#P3600_SHOW_GRIDS").val() == "-1" ){
    $("#main_lot .t20RegionBody").append("<a id='apply_btn' href='#' onClick='js_save_changes()' class='t20button' style=''>Update Date Code</a>");
    $("#P3600_CHILD").focus();
}
$("#main_lot .t20RegionBody").append("<a href='#' onClick='apex.submit(\"CLEAR_ALL\");' class='t20button'>Reset Form</a>");

//On Enter for child lot
$("#P3600_CHILD").keypress(function(event){
    if( event.keyCode == 13 && $(this).val().replace(" ", "") != "" ){
        apex.submit({request:'P3600_CHILD', showWait: true});
    }
});

});

I would like to show the message after the PL/SQL condition is evaluated, not in the JS. Any suggestion?