How make Nginx to show different pages for site down and apache down?

Question

How to make nginx to show page "Site down" when VirtualHost site opens for a minute or more (because of bad site code), and page "Apache down" when its ddos (server load average is >100) or Apache is down or something else?

Apache (2.2 FreeBSD) is running on 127.0.0.1 with a lot of VirtualHosts.

For now I have such nginx.conf:

    server {
    listen       80;
    server_name  localhost;
    error_page   404 /404.html;
    error_page   500 502 /apache_down.html;
    error_page   503 504 /site_down.html;

    location / {
            proxy_pass http://127.0.0.1:88;
            proxy_set_header Host $host;
            proxy_set_header X-Real-IP $remote_addr;
            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

            proxy_connect_timeout 5;
            proxy_read_timeout 6;
            proxy_send_timeout 6;
            proxy_intercept_errors off;
    }

    location = /404.html {
            root /usr/local/www/apache22/data;
    }
    location = /apache_down.html {
            root /usr/local/www/apache22/data;
    }
    location = /site_down.html {
            root /usr/local/www/apache22/data;
    }

The main problem, that when load average is very high, nginx returns "Site down" instead of "Apache down". Tried to use "try_files" but no luck.