Inherit parent taxonomy when creating child post

Im doing a register form that creates a new post when submitted, the new post is child of another programatically defined post so i want the new creted post to inherit the categories from its parent, i have this function working perfectly to do this when i create the post from the wordpress ui:

function set_parent_terms( $post_id, $post ) {
    if ( 'publish' === $post->post_status && $post->post_parent > 0 ) {
        $parent = get_post($post->post_parent);

        if(!empty($parent)){
            $taxonomies = get_object_taxonomies( $parent->post_type );
            foreach ( (array) $taxonomies as $taxonomy ) {
                $terms = wp_get_post_terms( $parent->ID, $taxonomy );
                if ( !empty( $terms ) ) {
                    $termArr = array_map(create_function('$obj', 'return $obj->term_id;'), $terms);
                    $tmp = wp_set_object_terms( $post_id, $termArr, $taxonomy, true );
                }
            }
        }
    }
}
add_action( 'save_post', 'set_parent_terms', 100, 2 );

But when i create the post using this code:

$horario = array(
          'post_title' => $user_first." ".$user_last,
             'post_content' => 'Nota:'.$horario_dj,
             'post_status' => 'publish',
           'post_author' => $new_user_id,
           'post_type' => 'horarios',
           'post_parent' => $horario_dj,
           'tax_input'    => $custom_tax
         );
    $new_post_ID = wp_insert_post($horario);

the function is not getting called, i already tried changing the “save_post” hook to everyhing there is related to creating a post, i even tried calling the function directly from the submit validation after the post is created with this code:

$post_object = get_post($new_post_ID);
set_parent_terms( $new_post_ID, $post_object )

but nothing seems to work, have any ideas?