We got in to a similar situation a few years ago. Still do work for the builder to this day. Hard to work with at first, and sometimes during the job but it pays off in the end.
The builders we do work for only a few jobs except for one since most do not put flat roofs on are million dollar plus builders. They are interested in quality work, of course it has to be at a fair price, but they are willing to pay more for a company who will be there when they need them and do great work and be done quickly.
The one started out just bid shopping, we weren't low by any means the house was large all flat roof. Two layer tear off, he questioned about our price, I explained it to him. Got the job, while the job was in progress, they wanted some flashing built for the stucco that was being replaced. Gave him a price, he signed and sent it right back needs it now! this kept going on this whole job managing this project was becoming a hassle, now the jobs done great. Two weeks later, needs collector heads and square downspouts, owner doesn't like the old ones. Boom heres a price granted it was very high since they collector heads were 35' from the ground. singed and sealed, need it done asap. Got it done quickly. Whew that jobs now complete. Move forward to a couple months later. Calls me up, need to come look at this house on the lake, complete remodel. Two sloped roofs flat roofs (one sloped the wrong way) and abortion of an entry way ect. Wants some numbers. Call the shingler, get him out there and figure our price on the flat, boom heres there numbers. Get a call a month later, wants to change some things, give him another price. Calls 2 weeks later, get me on the schedule. Got the job all completed in a good amount of time. Get a call 1 week after the job is done, owner doesn't like the front entry metal, willing to pay to replace. T&M I personally go out and do the work. 2nd best customer we work for, 1st is another home builder. Both pay on time, no hassles. the first homebuilder only hassled about a price once, and that was on cor-10 steel siding they wanted, it was expensive. The 2nd only hassled once, the initial contact with them.
Long but not so pointless post. This could very well be the best customer you have ever had. I would do like others have suggested, sit him down, explain this is why it cost so much, even giving him numbers of what it actually would have cost if you didn't do the extra work. Remember he called you to fix someone else mistake. That means they want you there. As long as you treat them fair you should be there. Like I said, upper ended home builders at least around here have a select few they use. Both to the ones I typed about only use the same people over and over. They want quality work, done quickly, at a decent price. Price should be and generally is last when your building homes like that.
Buying him a good lunch wouldn't hurt either
