Try this website to look at local state/county Veterans records for the state he was living. http://www.usgenweb.org/http://www.usgenweb.org/
Also try the MI state's government website for digital or online records. They may have them at the State Library, just not online yet. Try Google Books for a search of books about the Mexican War.
Another site I just found is http://www.archive.orghttp://www.archive.org Using their search box spell out the state name as today's MI was once Mich and the book title also may have full name and the search program won't find it using "MI".