Disable dragging of metaboxes in custom post types?

The current screen isn’t setup on the admin_init hook. That’s why global $current_screen and get_current_screen() don’t work.

Every admin page has a load-something hook that fires after the current screen is set up. Since you say this is for an events custom post type, you should use the load-post.php hook. So you’re code would look like:

function disable_drag_metabox() {
  if( 'events' === get_current_screen()->post_type ) {
    wp_deregister_script( 'postbox' );
  }
}
add_action( 'load-post.php', 'disable_drag_metabox' );

You can use the Query Monitor plugin to figure out what hooks fire on each page and in what order. It also does a lot of other cook stuff.