My first hope would be that the other subs will have your back. As a PM an estimator with many years under my belt, I wouldn't deny your change order costs unless they were grossly over the top, because what comes around goes around. They should realize that. I would insist on doing all future change orders on time and material. Making sure to negotiate, up front the labor rate for you to process the changes.
Likely the other subs will stab him in the back.
I have seen a fair number of change orders over the years. Usually they amount to a list of parts and labor with no indication of why the charges are outside the scope of the P.O. that was issued, or sometimes even what it was for.
I don't do much in the way of having to deal with the financial end of dealing with contractors anymore, but I sometimes get end users asking about the extra money they want. Sometimes it is stuff that was clearly in the original scope, other times it is not even clear what work was actually done. often there is no indication when the work was done either, or who did it.
But, as often as not, this is a problem created by both sides of the contract. if there is no clear understanding of how change orders are to be handled, it is almost always a big mess.
The thing most people hate is when the CO's get held until the end of the job and nothing is said about them until then and the guy footing the bill gets a bunch of change orders and no one knows what they are for or how they happened.
At the very least, before proceeding with changes, talk to the guy, then send the guy an email telling him based on his verbal instructions you are proceeding with the changes, and detailing those changes. It is not a perfect solution but it is a lot better than a bunch of expensive surprises.
I have to write change orders for stuff we do as well. It always works better if the guy footing the bill knows what is going on before the bill shows up.