Can my partner successfully renew a METV in Thailand after it expires and what are the potential immigration issues?

If you weren't anonymous I should give you advice

Back to back METV not a good idea

You can get 9 months out out of the 6 month METV with a last minute border bounce

There is no limit on how many you can buy back to back…but as a rule of thumb always remember there is a disconnect between the Embassy/Consulate and immigration

You will have to travel to your passport(s) country or a country you have permanent residence in to apply for the visa

Good Luck

He would first of all have to travel back to his home country to apply. Then the embassy in that country might or might not issue him with another METV. Then immigration might or might not deny him entry. I've seen reports of people trying to squeeze an additional 60 days out of an METV just before it expires being denied entry, so I would suggest his chances are not good. Probably best to leave Thailand until he has saved enough to apply for his desired visa. It's really not possible to live in Thailand any more on tourist/visa exempt entries