How to set default’ => $current_user->user_login

Your question is not very clear to me but here’s a version of your code which you may try:

<?php
global $current_user;
get_currentuserinfo();
return array(
    'author' => array(
        'label' => __('Author', 'ait-toolkit'),
        'type' => 'select-dynamic',
        'dataFunction' => 'AitItemCpt::fillAuthorMetabox',
        'default' => $current_user->ID, // or try changing it to $current_user->user_login if it doesn't work
        'capabilities' => true,
    )
);