List of all posts from current category on single portfolio page

Update your following part of code:

$term = get_queried_object();

$category_name =  $term->slug; //category slug

$args = array(
    'post_type'      => 'portfolio_page',
    'post_status'    => 'publish',
    'orderby'        => 'date',
    'order'          => 'DESC',
    'category_name'  => $category_name

);