When you apply for your Father's visit visa, The form shall include passport copy with RP stamp of you and your wife. Without your Wife's RP stamp attached to the application form, the visit visa of your father will not be approved.

Once your father arrives here, He will given an entry of 1 month only. Then he will have to undergo medical at the medical commission. Then you can get his visa extended by 5 months. So total six months on each visit.