Romney is somewhat of a native son, but the connection isn’t strong. I do recall that in 2004, George W. Bush nearly won Michigan, losing by only 3% of the vote. It’s difficult, but not impossible.
Mitt was born in Detroit, so that would make him literally a native son. That doesn’t necessarily mean he will carry the state because of it. The first President Bush was born in Massachusetts but did not carry the state in 1988 or 1992. The second President Bush was born in Connecticut and lost the state both times.