If he has not cashed the money order yet then technally you have not held up your end of the deal. You have to allow time for it to get to him, if you insist on sending a money order yo must also understand that this is the slowest way of doing business on e-bay.
I'm sorry but i think that for you to go ahead and give negative feed back before you even find out if he cashed the moneyorder makes you no better than a seller on e-bay who dosen't deliver the products offerd. Next time you need to be more patient.
Hell, he might be on some other forum right now starting a thread about the asshole who never sent his payment but left negative feedback.