In the episodes that have aired, we have not heard Ned's last name. He is often referred to as "Pie Boy" or "The Pie Maker" and then Ned.

However, based on Dwight Dixon's pocket watch collection, where his own watch is marked DD and Chuck's father's watch is marked CC, we can surmise that Ned's father's last name (and thus Ned's last name) starts with N (in order to make the watch be NN}

I don't think we have ever been told his last name!!!

Nope... never heard it. It will probably always be a mystery.