I'm certainly not an expert in these matters, but, my two cents:
-This has probably turned into an issue of pride for him, more than anything having to do with you. There's a natural tendency in situations like these, when confronted with the issue, to dig in and insist on being right. I think that this tendency is even stronger in a d/s relationship, where your Master probably does not want to show any weakness or vacillation in front of you. This is understandable, if a bit....pig-headed.
Second, in my opinion, patience is one of the greatest virtues a sub can have. Give him enough time, and your Master will probably reconsider the situation. It's been seven years, and I'm sure he doesn't want to lose this-give him time, trust him, and things may work out fine.