I believe this is a strong indication that he was not MIA/KIA during WWII. This assumes, of course, that Michigan was listed as his place of residence during his time in the Corps. If he had given them a different residence, he might be in the database for another state.
It appears you have not gotten his personnel record? I would think that would tell you what happened to him while he was in the service, including the nature of his discharge. You should be able to get his record through NPRC. They had a disastrous fire in 1973, but Marine Corps records were not affected (according to info at the NPRC web site). Try this web page for further info re acquisition of records: