This is easy... his main home is MI and probably his DL states that.   His absence, so be it in Jail/Prison/College etc. etc.. is a temporary absence.... IF he worked  or made any taxable income while on his absence in WV.. he would file as a NON RESIDENT for that state.. but for sure he is a RESIDENT for the year in MI.  (NOT PART YEAR in either state)   

I have had clients in the past under the same type of situation... and Prison is not a residence.. its a correction center... like college is an educational center.  ( think of it that way and it will make sense to you)   Good Luck!