Sounds like there is either a routing issue between the user and your server/datacenter, or his IP address is being blocked by the firewall. Ask the user for his IP and try the following:
1) Do a tracert from the server back to his IP address.
2) Ask the user to do a tracert to your server (so you can see where the problem is taking place)
3) Make sure the user's IP has not been blacklisted by your server's firewall.
Just to note, I was going to run some tracert's on the website in your signature, but that domain hasn't been registered. Not sure if you let it expire or what. I was going to go ahead and register it for you and transfer it over to you, but I don't know you and don't know that you'll reimburse me for the cost. I'd get on that right away if I were you, so somebody doesn't register your domain name, which you apparently let expire, or never registered.