How to load an assets based on custom field value?

You would want to use the wp_enqueue_scripts action, and within there, instead of custom_field you would use get_post_meta, like this:

function wpse217823_enqueue_per_custom_field(){
    global $post; 
    $layout = get_post_meta( $post->ID, 'layout', true ); 
    if( $layout == ... ){
        wp_enqueue_style( 'my-custom-style-here' );
    }
}
add_action( 'wp_enqueue_scripts', 'wpse217823_enqueue_per_custom_field' );