Big Al you are exactly what's wrong with today's contractors...
Listen to the others son, talk to the HO, but don't try to muscle them into paying for what might be your mistake.
Ask of them to help you find a way out, they will respect you for not bailing.
I think you are missing what I am saying. I am not saying stick it to the customer. If there are legitimate change orders to be paid than he should be paid for them. where the cost over runs are due to poor biding than that is the contractors problem.
Some of the Items that he listed may of exceeded the scope of work.
The main thing is to be honest and address the situation head on.
Early on I got in a simular situation and my client gave me a two year loan to make it right, if addresed head on a solution will be found.