ok, perhaps just like this :
Just put it in custom JS of theme options
jQuery(document).ready( function ($) {
$( "#mobile-menu ul.sub-menu" ).not( ".current-menu-ancestor > ul.sub-menu" ).hide();
});
with this in custom css to be more visible :
#mobile-menu .menu li.current-menu-ancestor > a, #mobile-menu .menu ul li.current-menu-ancestor > a, #mobile-menu .menu ul li.current-menu-item > a {
background-color: #313131!important;
}