As long as you can explain to immigration why you have a work extension from Bangkok but are living in Phang Nga. You won't be able to do a 90 day report in Bangkok unless you have a TM30 from Bangkok.
7 days might not be enough time to apply for a visa, if you mean you will only be there for 7 days. If you have a Thai wife or Thai child, you can get a visa based on that.
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.