What Not To Use
Do not use hisn for HIS; hern for HERS;
ourn for OURS; yourn for YOURS;
theirn for THEIRS; who's for WHOSE.
These possessive forms do not take the apostrophe.
I think that most of us would consider such usage particular to parts of the deep south. It may even sound ignorant to certain (northern) ears. But it must, at one time, have been more widespread before our hard-working school-marms stamped it out.