As explained before:
DoS = Denial of Service.
DDoS = Distributed Denial of Service.
Both effectively consume your bandwidth to the point where your server canot handle any more incoming connections, and you are thus "denied service".
DoSes are a piece of cake. DDoSes are a pain in the arse because it could be thousands of IP addresses, in which case, its a tedious and repetetive job to sort it (if you can even get into some kinda ACP).
You are usually targeted for a reason. People don't usually random-attack websites. You indicated that you have been in contact with the offender. I suggest speaking with him and trying to resolve the problem. You can't really "explain" how to stop these attacks. It requires a whole bunch of "know-how" and "trying" to put a stop to it.
