Loading scripts & styles from a meta box callback function

Your problem is adding the scripts, styles inside the class, and because the class instance is created when the hook add_meta_box is fired, at that time the wp_enqueue_script/style are already finished.

A solution for you is adding the scripts, styles outside the function and the class. And because meta boxes are used only at editing pages, you can do as the following:

// the editing pages are actually post.php and post-new.php
add_action('admin_print_styles-post.php', 'custom_js_css');
add_action('admin_print_styles-post-new.php', 'custom_js_css');

function custom_js_css() {
    wp_enqueue_style('your-meta-box', $base_url . '/meta-box.css');
    wp_enqueue_script('your-meta-box', $base_url . '/meta-box.js', array('jquery'), null, true);
}

Actually, I wrote a meta box class for WP, that uses another approach. Instead of writing form class for form fields like you did, my class is a wrapper for meta boxes. If you interested in, it’s worth to take a look.