Nothing will change if he doesn't want to switch the reason for his extension. In the future, if he decides to change, all he has to do is make sure his bank account has at least 400,000 baht around October and stays at that level until he applies for his extension, and instead of applying for the extension based on employment, he would apply based on being married to a Thai. It won't affect his work or work permit at all. In fact if he's still working at that time, he can use proof of his salary over 40,000 baht per month and won't even need the bank balance.
How close are you to the extension expiring? I've seen reports of 3 weeks for a new US passport. He can still apply for an extension even if he goes onto overstay, he'll just have to pay the fine. It might be worth it to order the new passport immediately, and as soon as it arrives go and apply for the extension and pay the overstay fine.
I'm not sure what you mean by 4 weeks, but if you can show you had the money last year and you can show 12 months of international transfers into your account, you should be fine.